GIFT 

OF 

U.  S.  Department  of  Agriculture,  Forest  Service 

FOREST  PRODUCTS  LABORATORY 

In  cooperation  with  the  University  of  Wisconsin 

MADISON,  WISCONSIN 


WOOD  IN  AIRCRAFT  CONSTRUCTION 


Approved  Copy  Filed 


I*  ?f 
~ 


MAU-i  LIBRARY-AGRICULTURE  DEM". 


AIRCRAFT  DESIGN  DATA.     NOTE  NO.  12. 

WOOD  IN  AIRCRAFT  CONSTRUCTION. 

[Prepared  by  the  Forest  Products  Laboratory,  Forest  Service,  TJ.  S.  Department  of  Agriculture.] 


CONTENTS. 

Page. 


Mechanical  and  physical  properties — 

Variability  of  the  strength  of  wood 

Wood  nonhomogeneous 

Variation  of  strength  with  locality  of  growth 

Variation  of  strength  with  position  in  tree 

Variation  of  strength  with  rate  of  growth 

Variation  of  strength  with  amount  of  summmvood 

Variation  of  strength  with  specific  gravity 

Variation  of  strength  with  moisture  content 

Defects  affecting  strength 


-Xl/^cUIH.H-H^^l'l^JiSl'ii , ** 

Diagonal  and  spiral  grain 11 

Knots 21 

Pitch  pockets 21 

Compression  failures  and  "cross  breaks" 22 

Braslmess 22 

Decay 22 

Internal  or  initial  stresses  in  wood 22 

Wood  fibers  under  stress  in  the  tree 22 

Internal  stresses  produced  during  drying 25 

Initial  stresses  produced  in  assembling 25 

Working  stresses  for  wood  in  aircraft  construction 25 

Nature  of  loading "  27 

Tensile  strength 27 

Torsional  strength 28 

Shrinkage 28 

Suitability  of  various  American  woods  for  aircraft 34 

Construction ." 

Conifers 

Hardwoods 

Storage  and  kiln  drying 40 


Rules  for  piling  lumber 

Kiln  drying  of  wood 

Advantages  of  kiln  drying 

The  elimination  of  moisture  from  wood 

Three  essential  qualities  of  a  dry  kiln 

Defects  due  to  improper  drying 

Case  hardening  and  honeycombing 

Collapse 


40 
41 
41 
41 
42 
42 
42 
44 

Brashness 45 

Methods  of  testing  conditions  during  drying 45 

Preliminary  tests 46 

Current  tests 46 

Final  tests 49 

Specifications  for  kiln  drying  lor  aircraft  stock. 50 

Treatment  of  wood  after  removal  from  kiln 55 

Changes  of  moisture  in  wood  with  humidity  of  air 50 

Veneer  and  plywood 57 

Veneer 57 

Plywood 59 

Properties  of  wood  parallel  and  perpendicular  to  the  grain .        5<i 

Plywood  panels  vs.  solid  panels 61 

Symmetrical  construction  in  plywood 61 

Direction  of  grain  of  adjoining  plies 62 

Effect  of  moisture  content 62 

Shrinkage  of  plywood 63 

Effect  of  varying  the  number  of  plies 63 

Effect  of  varying  the  ratio  of  core  to  total  thickness 63 

Species  of  low  density  for  cores 64 

Plywood  test  data , 64 

Riveted  joints  in  plywood 70 

Joints  in  individual  plies 80 

Joints  extending  through  the  entire  thickness  of  plywood .       81 

Thin  plywood 82 

98257— 19— No.  12 1 


Page. 

Veneer  and  plywood— Continued. 
Ply  wood— Continued . 

Woven  plywood 83 

Specification  for  water-resistant  veneer  panels  or  plywood.  83 

Glues  and  gluing 86 

Hide  and  bone  glues 86 

Testing  of  hide  glue 86 

Precautions  in  using  hide  glue 90 

Liquid  glues 91 

Marine  glues 91 

Blood  albumen  glues 91 

Casein  glues 92 

Instructions  for  use 92 

Equipment 92 

Preparation  of  glue 92 

I'roportions  of  dry  glue  and  water 92 

Mixing  the  glue 92 

Consistency  of  glue [[  92 

Application  and  use  of  glue 94 

Directions  for  mixing  Certus  glue 95 

Directions  for  mixing  Napco  glue 95 

'  *     Directions  for  mixing  Casco  glue 95 

Directions  for  mixing  Perkins  Waterproof  Casein  Glue 96 

Aircraft  parts 96 

Laminated  construction % 

Wing  beams 98 

Results  of  various  beam  tests 98 

General  conclusions 100 

Beam  splices 100 

Struts 108 

Methods  of  test 103 

Tests  on  standard  J-l  struts 103 

Tests  on  rejected  J-l  struts 104 

Tests  on  standard  de  Havilland  struts 105 

Tests  on  rejected  de  Havilland  struts 106 

Tests  on  standard  F5-L  struts 106 

Two  noninjurious  test  methods  for  inspecting  struts 106 

Discussion  of  noninjurious  test  methods 109 

Comparison  of  two  test  methods  by  actual  trials 110 

Miscellaneous  strut  tests 112 

Tests  on  struts  stream  lined  with  plywood 1 12 

Tests  on  struts  covered  with  bakelized  canvas 112 

Effect  of  taper  on  the  strength  of  struts 113 

Design  and  manufacture  of  built-up  struts 113 

Use  of  materials  of  different  density 114 

Possibility  of  using  defective  material 115 

Possibility  of  warping  or  bowing llfi 

Conclusions 117 

Wing  ribs 119 

Tests  on  DH-4  wing  ribs 124 

Tests  on  SE-5  wing  ribs 125 

Tests  on  HS  wing  ribs 126 

Tests  on  F5-L  wing  ribs 128 

Tests  on  15-foot  wing  ribs 13& 

Tests  on  elevator  or  aileron  spars 1 134 

Tests  on  aircrat't  engine  bearers .  ^ 136 

Tests  on  bakelized  canvas  (micarta) 14C 

Treatments  for  preventing  changes  in  moisture 140 

Instructions  for  applying  aluminum  leaf  to  aircraft  propel- 
lers   112 

Appendix 1*7 

The  determination  of  moisture  content  In  wood .  147 

The  determination  of  specific  gravity  of  wood 147 


415834 


:'Alkt?BAFT  DESIGN  DATA.  Note  12. 


MECHANICAL  AND  PHYSICAL  PROPERTIES. 

Wood  differs  from  other  structural  materials  in  a  great  many  ways,  and  the  maximum 
efficiency  in  its  use  demands  a  thorough  knowledge  of  the  properties  of  wood  and  of  the  factors 
which  influence  these  properties.  In  the  following  general  discussion  an  attempt  is  made  to 
explain  the  principal  causes  for  the  wide  variations  found  in  the  strength  of  wood  and  to  show 
how  these  variations  may  be  largely  eliminated  in  any  group  of  material  by  proper  specifi- 
cation and  inspection. 

VARIABILITY  OF  THE  STRENGTH  OF  WOOD. 

WOOD   NON-HOMOGENEOUS. 

Wood  is  exceedingly  variable  as  compared  with  other  structural  materials.  This  vari- 
ability is  due  to  a  number  of  factors,  heretofore  not  well  understood.  For  that  reason  any 
judgment  of  the  strength  of  a  piece  was  felt  to  be  uncertain.  The  causes  for  variations  in 
the  properties  of  wood  can  now  be  given  and  their  effects  anticipated  within  reasonable  limits. 

VARIATION   OF   STRENGTH   WITH   LOCALITY   OP   GROWTH. 

In  some  cases  the  locality  of  growth  has  an  influence  on  the  strength  of  the  timber.  For 
example,  tests  show  a  marked  difference  in  strength  between  the  Rocky  Mountain  and  coast 
types  of  Douglas  fir  in  favor  of  the  coast  type. 

This  influence  of  locality  is  usually  overestimated.  Different  stands  of  the  same  species 
grown  in  the  same  section  of  the  country  may  show  as  great  differences  as  stands  grown  in 
widely  separated  regions,  so  that  as  a  rule  locality  of  growth  can  be  neglected. 

VARIATION   OF   STRENGTH   WITH   POSITION   IN   THE   TREE. 

In  some  instances  specimens  from  different  parts  of  the  same  tree  have  been  found  to 
show  considerable  difference  hi  strength.  In  most  cases,  however,  the  wood  of  the  highest 
specific  gravity  has  the  best  mechanical  properties  regardless  of  its  position  in  the  tree. 
Where  this  is  not  the  case,  the  toughest  or  most  shock-resistant  material  is  found  near  the 
butt.  Above  a  height  of  10  or  12  feet  variations  of  mechanical  strength  correspond  to  the 
variations  of  specific  gravity.  Some  variations  with  position  hi  cross  section  or  distance  from 
the  pith  of  the  tree  have  been  found  which  could  not  be  entirely  accounted  for  by  differences 
in  specific  gravity. 

VARIATION    OF   STRENGTH   WITH   RATE   OF   GROWTH. 

Strength  is  not  definitely  proportional  to  rate  of  growth,  either  directly  or  inversely. 

Timber  of  any  species  which  has  grown  with  exceptional  slowness  is  usually  below  the 
average  of  the  species  in  strength  values. 

Among  many  of  the  hardwood  species,  material  of  very  rapid  growth  is  usually  above 
the  average  in  strength  properties.  Notable  exceptions  to  this  are  found,  however,  and  rapid 
growth  is  no  assurance  of  excellence  of  material  unless  accompanied  by  relatively  high  spe- 
cific gravity.  This  is  particularly  true  of  ash. 

In  the  coniferous  species,  material  of  very  rapid  growth  is  very  likely  to  be  quite  brash 
and  below  the  average  strength. 


Note  12.  AIRCRAFT  DESIGN  DATA. 


VARIATION   OF   STRENGTH   WITH   AMOUNT   OF   SUMMER   WOOD. 

• 

In  many  species  the  proportion  of  summer  wood  is  indicative  of  the  specific  gravity, , and; 
different  proportions  of  summer  wood  are  usually  accompanied  by  different  specific  gravities 
and  strength  values.  However,  proportion  of  summer  wood  is  not  a  sufficiently  accurate 
indicator  of  strength  to  permititsuse  as  the  sole  criterion  for  the  acceptance  or  rejection  of  airplane 
material.  After  some  practice  one  should  be  able,  through  observation  of  the  proportion  of 
summer  wood,  to  decide  whether  any  particular  piece  is  considerably  below,  considerably  above, 
or  near  the  required  specific  gravity.  Caution  must  be  observed  in  applying  this  to  ash,  and  per- 
haps to  other  hardwoods,  since  rapid-growth  ash  is  sometimes  very  low  in  specific  gravity 
in  spite  of  a  large  proportion  of  summer  wood.  In  such  cases  careful  examination  will  show 
that  the  summer  wood  is  less  dense  than  usual. 

VARIATION   OF   STRENGTH   WITH   SPECIFIC    GRAVITY. 

A  piece  of  clear,  sound,  straight-grained  wood  of  any  species  is  not  necessarily  a  good  stick 
of  timber.  To  determine  the  quality  of  an  individual  stick  by  means  of  mechanical  tests  is 
extremely  difficult,  because  the  variations  in  strength  of  timber  due  to  variations  in  moisture 
content,  temperature,  speed  of  test,  etc.,  are  so  great.  Furthermore,  a  test  for  one  strength 
property  does  not  always  indicate  what  the  other  properties  of  the  timber  are.  Without  actual 
and  complete  tests,  the  best  criterion  of  the  strength  properties  of  any  piece  of  timber  is  its 
specific  gravity  or  weight  per  unit  volume,  weight  being  taken  when  the  wood  is  completely 
dry  and  volume  when  the  wood  is  at  some  definite  condition  of  seasoning  or  moisture  content. 
Specific  gravity  based  on  oven-dry  volume  is  greater  than  that  based  on  the  volume  at  any 
other  moisture  condition  in  proportion  to  the  shrinkage  which  takes  place  as  the  moisture 
is  driven  out  and  the  wood  is  reduced  to  the  oven-dry  condition. 

Accurate  determinations  made  on  seven  species  of  wood,  including  both  hardwoods  and 
conifers,  showed  a  range  of  only  about  4£  per  cent  in  the  density  of  the  wood  substance,  or 
material  of  which  the  cell  walls  is  composed.  Since  the  density  of  wood  substance  is  so  nearly 
constant,  it  may  be  said  that  the  specific  gravity  of  a  given  piece  of  wood  is  a  measure  of  the 
amount  of  wood  substance  contained  in  a  unit  volume  of  it.  Very  careful  analyses  based  on 
a  vast  amount  of  data  have  shown  that  wood  of  high  specific  gravity  has  greater  strength  than 
that  of  low  specific  gravity.  Some  fairly  definite  mathematical  relations  between  specific 
gravity  and  the  various  strength  properties  have  been  worked  out.  Some  of  the  strength 
properties  (strength  in  compression  parallel  to  grain  and  modulus  of  elasticity)  vary  directly 
as  the  first  power  of  the  specific  gravity;  others,  however,  vary  with  higher  powers  of  the 
specific  gravity,  i.  e.,  the  strength  property  changes  more  rapidly  than  the  specific  gravity,  a 
10  per  cent  increase  of  specific  gravity  resulting  in  an  increase  in  the  strength  properties  of 
15  per  cent  to  even  30  per  cent. 

The  rate  of  change  in  strength  with  changes  of  specific  gravity  is  usually  greater  in  indi- 
vidual specimens  of  a  single  species  than  in  the  averages  for  a  number  of  species.  This  is 
illustrated  by  a  comparison  of  figures  1  and  2.  Figure  1  indicates  that  the  modulus  of  rupture 
varies  as  the  5/4  power  of  the  specific  gravity  when  various  species  are  considered,  while  figure 
2  indicates  that  the  relation  of  the  crushing  strength  of  individual  specimens  of  white  ash  varies 
as  the  3/2  power  of  the  specific  gravity.  The  modulus  of  rupture  of  spruce  and  of  numerous 
other  species  has  been  found  to  vary  as  the  3/2  power  of  the  specific  gravity.  Shock-resisting 
ability  and  other  important  properties  vary  as  even  higher  powers  of  specific  gravity.  If  an 
important  airplane  part  is  from  wood  10  per  cent  below  the  specific  gravity  given  in  the  speci- 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


fications,  it  will  not  be  just  10  per  cent  but  at  least  14.5  per  cent  inferior  and  perhaps  more, 
depending  on  which  particular  property  is  of  greatest  importance  in  the  part  in  question.  If 
the  specific  gravity  is  20  per  cent  low,  the  inferiority  will  not  be  less  than  28.4  per  cent.  The 


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SPECIFIC   GRAVITY 


Fig.  1. — Relation  between  the  modulus  of  rupture  and  specific  gravity  of  various  American  woods. 

lighter  pieces  of  wood  are  usually  exceedingly  brash,  especially  when  dry.  The  importance  of 
admitting  no  material  for  airplane  construction  of  lower  specific  gravity  than  given  in  the 
specifications  is  evident. 

. 

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List  of  species  and  reference  numbers  for  figure  1. 
HARDWOODS. 


Species. 

Locality. 

Reference 
No. 

Species. 

Locality. 

Reference 
No. 

Alder  red 

Washington  

30 

Hickory  —  Continued  . 

Ash: 

Pignut  

Pennsylvania 

160 

Biltmore     

Tennessee  

91 

Do  

West  Virginia 

161 

Black  

Michigan  

60 

Shagbark  

Mississippi  

140 

Do              .    ... 

Wisconsin        

70 

Do                

Ohio 

159 

Blue           

Kentucky  

!)() 

Do  

Pennsylvania 

143 

Green               

Louisiana  

93 

Do  

West  Virginia 

153 

Do              .    ... 

Missouri    

100 

Water                

M  ississippi 

Pumpkin         

do                   

79 

Holly,  American 

Tennessee 

Q7 

White         

Arkansas  

106 

Hornbeam  

do 

149 

Do           

New  York  

128 

Laurel,  mountain  

do 

145 

Do              

West  Virginia   

83 

Locust: 

Aspen              

Wisconsin  

23 

Black.  

do 

158 

Largetooth     

..do  

20 

Honey    

Indiana 

169 

Basswood                     .  .  . 

Pennsylvania    

12 

Madrona                     .... 

California 

101 

Do           

Wisconsin  

5 

Do  

Oregon 

128a 

Beech                  

Indiana  

110 

Magnolia  .        

I  ouisiana 

fil> 

Do                      

Pennsylvania  

98 

Maple  : 

Birclr 

Oregon  

Washington 

58 

Paper               

Wisconsin    

73 

Red          

Pennsylvania 

69 

Sweet   

Pennsylvania  

129 

Do  

Wisconsin 

92 

Yellow   

..do  

107 

Silver  

do 

56 

Do           

Wisconsin    

103 

Sugar        

Indiana  ' 

104 

Buckeye  yellow 

Tennessee        

9 

Do              

Pennsylvania 

108 

Buckthorn,  cascara  .  .  .  . 

Oregon  

84a 

Do  

Wisconsin 

124 

Butternut         

Tennessee  

27 

Oak: 

Do  

Wisconsin  

21 

Bur  

do 

125 

Chinquapin,  western  .  .  . 

Oregon    

48b 

California  black  .... 

California 

80 

Cherry- 

Canyon  live  

do 

163 

Black  

Pennsylvania  

72 

Chestnut  

Tennessee 

121 

Wild  red  

Tennessee  

24 

Cow    

Louisiana 

133 

Chestnut             .     ... 

M  aryland 

46 

Laurel        .       

do 

116 

Do    

Tennessee  

40 

Post  

Arkansas 

130 

Cotton  wood,  black  ...    . 

Washington  

6  ' 

Do    

Louisiana 

137 

Cucumber  tree 

Tennessee      

59 

Red             

Arkansas 

119 

Dogwood  : 

Do  

Indiana 

118 

Flowering  

do     

151 

Do      

Louisiana 

117 

Western  

Oregon  

125a 

Do  

Tennessee  

97 

Elder,  pale  

.   do  

69a 

Highland  Spanish  .  . 

Louisiana 

94 

Elm: 

Lowland  Spanish  .  .  . 

.  do  

142 

Cork  

Wisconsin,     Marathon 

126 

Swamp  white  

Indiana       

150 

County. 

Tanbark  

California 

115 

Do    

Wisconsin,     Rusk 

Water         ... 

Louisiana 

111 

County. 

White  

Arkansas    

132 

Slippery  

Indiana  

102 

Do    

Indiana 

138 

Do      

Wisconsin    

74 

Do       

Louisiana       Richland 

136 

White 

Pennsylvania      

55 

Parish 

Do  

Wisconsin  

53 

Do  

Louisiana,  Winn  Parish 

131 

Greenheart  

165 

Willow  

Louisiana  

109 

Gum: 

Yellow  

Arkansas  

122 

Black  

Tennessee  

68 

Do    

Wisconsin 

105 

Blue  (Eucalyptus) 

California  

147 

Osage  orange 

Indiana 

164 

Cotton       .   . 

Louisiana            .    .    . 

76 

Poplar     yellow    (tulip 

Tennessee 

35 

Red  

Missouri  

54 

tree) 

Hackberry  

Indiana        

90 

Rhododendron,  great. 

do 

85 

Do  

Wisconsin  

78 

Sassafras  

do  

51 

Haw,  pear  

do  

146 

Serviceberry    

do      

156 

Hickory  : 

Silverbell  tree  

do  

49 

Big  shellbark  

Mississippi  

135 

Sourwood 

do 

89 

Do  

Ohio  

154 

Sumac,  staghorn  

Wisconsin  

61 

Butternut  

do  

139 

Sycamore  

Indiana    

63 

Mockernut  

Mississippi  

144 

Do 

Tennessee  .  ... 

65 

Do  

Pennsylvania    .  .  . 

159 

Umbrella  Eraser 

do 

45 

Do  

West  Virginia 

155 

Willow 

Nutmeg  

Mississippi  

112 

Black  

Wisconsin  

11 

Pignut  

do  

148 

Western  black 

Oregon    .   ... 

43a 

Do  

Ohio  

157 

Witch  hazel 

Tennessee 

114 

AIECEAFT  DESIGN  DATA. 


Note  12. 


List  of  species  and  reference  numbers  for  figure  1 — Continued. 

CONIFERS. 


,  -  - 

Species. 

• 

Locality. 

Reference 
No. 

Species. 

Locality. 

Reference 
No. 

Cedar: 
Incense         

California          

26 

Pine  —  Continued. 
Lodgepole  

Montana,    Granite 

41a 

Western  red 

Montana                  .    ... 

2 

County. 

Do      

Washington 

10 

Do  

Montana,         Jefferson 

40a 

•  ••;   WJiite  

Wisconsin  

1 

County. 

t  'v  press,  bald 

Louisiana 

62 

Do              

Wyoming  

34 

Douglas  fir 

California 

45a 

Lonirleaf 

Florida   ... 

123 

Do  

Oregon     

67a 

Do  

Ix)uisiana,Lake  Charles. 

113 

Do  

Washington,    Chelialis 

46a 

Do        

Louisiana,  Tangipahoa 

96 

Do  

County. 
Washington,     Lewis 

75 

Do 

Parish. 
Mississippi 

95 

County. 

Norway  

Wisconsin  

57 

Do  

Washington  and   Ore- 

67 

Pitch        

Tennessee    .  .   . 

71 

gon. 

Pond                

Florida 

86 

Do... 

Wyoming  

84 

Shortleaf 

Arkansas 

77 

Fir: 

Sugar    

California  

22 

A  Ipine  

Colorado 

4 

Table  Mountain 

Tennessee 

82 

Amabilis  

Oregon  

39 

Western  white 

Montana 

42 

Do  

Washington  

18 

\Vestern  yellow 

Arizona 

19 

Balsam  

Wisconsin  

14 

Do 

California 

37 

Grand  

Montana.  .       

36 

Do 

Colorado 

41 

Npble  : 

Oregon  

16 

Do 

Montana 

32 

White  

California  

17 

White 

Wisconsin 

25 

Hemlock: 

Redwood      

California,  Albion 

28 

Black  

Montana  

47 

Do 

California  Korbel 

13 

Eastern  

Tennessee  

52 

Spruce' 

Do  

Wisconsin 

15 

ETigelmann 

g 

Wjestern  

Washington  

50 

Do 

Colorado    San  Miguel 

3 

Larch,!  western  

Montana 

84 

D6  -  

Washington  

64 

Red 

44 

Pine:  i 

Do  

Tennessee  

29 

Ciiban  

Florida  

127 

White 

7 

Jack  

Wisconsin  

43 

Do 

Wisconsin 

38 

Jeffrey  

California  

33 

Tamarack 

do 

81 

LdbloUy  

Florida  

88 

Yew,  western 

134 

Lqdgepole  , 

Colorado  

31 

!    Do  

Montana      Gallatin 

35a 

:  

1    
_  1  

County. 

. 
n 

62 

6ft 

8 

. 


• 


• 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


7 


The  minimum  strength  values  which  may  be  expected  of  a  particular  lot  of  lumber  can 
be  raised  a  good  deal  by  eliminating  a  relatively  small  portion  of  the  lighter  material.  This 
lightweight  material  can,  as  a  rule,  be  detected  by  visual  inspection.  In  order  to  train  the 
visual  inspection  and  to  pass  judgment  on  questionable  individual  pieces,  frequent  specific 
gravity  determinations  are  necessary. 


8000 


7000 


6000 


5000 


-j   4000 

I 


3000 


2000 


1000 


w 

R 

MAXIf 
SIR 
SPECI 

BASED 
SPECK 
A     SRE 

HITE   ASH 

ELATION     OF 

1UM  CRUSHING 
ENGTH    TO 
FIC  GRAVITY 

D 

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(ENS    TESTED     IN 
EN      CONDITION 

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.1                .2                .3                4                .5                .6                .7                .8                .9                 1 
SPECIFIC    GRAVITY  -OVEN  DRY 

•ASID     ON    OREEN      VOLUME 

Fig.  2. 

A  specific  gravity  determination  is  relatively  simple  to  make,  and  it  is  probably  a  better 
criterion  of  all  the  qualities  of  the  piece  than  any  single  mechanical  test  which  is  likely  to  be 
applied;  also  the  specific  gravity  determinations  need  no  adjustment  such  as  would  be  neces- 
sary on  account  of  the  varied  conditions  of  a  mechanical  test. 


;l 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


VARIATION    OF    STRENGTH    WITH    MOISTURE    CONTENT. 

When  a  piece  of  green  or  wet  wood  is  dried,  no  change  in  mechanical  properties  takes  place 
until  the  fiber-saturation  point  is  reached.*  The  changes  beyond  this  point  for  small  test 
specimens  free  from  defects  and  very  carefully  dried  are  illusl rated  in  figures  3  and  4.  These 


1,000 


300 


15         20        25        30        35         40        45         50         55 
MOISTURE-PER    CENT    OF    DRY    WEJGHT. 


60 


65    „  70 


Fig.  3. — Relation  between  the  stiffness  (modulus  of  elasticity)  in  binding  and  moisture  content,  for  three  species. 

figures  show  that  the  moisture  content  at  the  fiber-saturation  point  differs  for  different  species. 
It  will  be  noted  that  the  influence  of  moisture  is  smaller  in  tests  of  shearing  strength  and 
compression  perpendicular  to  the  grain  than  in  bending  and  compression  parallel  to  the  grain. 

*  The  eucalypts  and  some  of  the  oaks  are  exceptions  to  this  rule. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


9 


Furthermore,  there  is  no  definite  break  at  or  near  the  fiber-saturation  point  in  the  moisture- 
strength  curves  for  shear  and  compression  perpendicular  to  the  grain.  In  the  case  of  shear 
this  failure  to  show  large  increases  in  strength  is  probably  due  to  checks  which  form  as  the 
material  dries. 


IftOOO 


• 
i  srfT 

bllB 
Xfil  Off*  10 


Bigdniii 


ma—  .a  .si1* 


aoqu 


6  5          10         -15         20        25         30        35         40        45 

MOISTURE- PER   CENT   OF  DRY    WEIGHT. 

Fig.  4. — Comparison  of  the  relation  between  strength  and  moisture  content  for  red  spruce  in  various  kinds  of  tests. 
(The  lowest  curve  is  for  compression  at  right  angles  to  grain.) 

The  moisture  content  at  the  fiber-saturation  point  varies  not  only  with  the  species  but 
with  different  specimens  of  the  same  species.  The  percentage  change  of  strength  which  results 
from  a  given  change  of  moisture  also  varies  with  the  species  and  with  individual  specimens  of 
the  species. 


bllf. 


fJT^  «i'  '.[jlti 


10 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


The  form  of  the  curves  shown  in  figures  3  and  4  applies  only  to  small  clear  pieces  very 
carefully  dried  and  having  a  practically  uniform  moisture  content  throughout.  If  the  mois- 
ture be  unequally  distributed  in  the  specimen,  as  is  the  case  of  large  timbers  rapidly  dried  or 
of  "case-hardened"  pieces,  the  outer  shelj  may  be  drier  than  the  fiber-saturation  point  while 
the  inside  still  contains  free  water.  The  resulting  moisture-strength  curve  wiJU  be  higher  than 
the  curve  from  carefully  dried  pieces  and  wil,l  be  so  rounded  off  from  the  driest  to  the  wettest 
condition  as  to  obscure  entirely  the  fiber-saturation  point  (see  fig.  5). 

The  increase  in  strength  which  takes  place  in  drying  wood  depends  upon  the  specimen 
and  upon  the  care  with  which  the  drying  process  is  carried  out.  Furthermore,  while  the  strength 
of  the  fibers  is  no  doubt  greatly  increased  by  any  reasonable  drying  process,  the  increase  of  the 
strength  of  a  piece  of  timber  taken  as  a  whole  may  be  very  much  less.  Knots  are  more  or  less 
loosened,  checking  takes  place,  and  shakes  are  further  developed.  In  large  bridge  and  building 
timbers  these  effects  are  so  great  that  it  is  not  considered  safe  to  figure  on  such  timbers  having 
greater  strength  when  dry  than  when  green.  When  the  pieces  are  small  and  practically  free 


|iwoo 
^ )  i.ooo 

10  10,000 
9 

§  9.000 
<4  6,000 

t  7,000 

i 

k  6.006 

3> 


5.000 


>%. 


10        ft 


30       JS        40        4S'        SO       SS       60       65 
MOISTURE      PERCENT,  or    DRV   WEIOH1. 


70         75         00         86,       90     OVtRSO 


S9 


Fig.  5. — Effect  of  case-hardening  upon  the  form  of  the  moisture-strength  curve  in  bending  tests.    The  upper  curve 
is  from  case-hardened  specimens,  the  lower  curve  from  uniformly  dired  specimens. 

from  defects,  as  in  airplane  construction,  proper  drying  with  careful  control  of  temperature 
and  humidity  increases  the  strength  of  material  very  greatly.  In  whatever  way  wood  is  dried, 
upon  its  being  resoaked  and  brought  back  to  the  original  green  or  wet  condition  it  is  found  to 
be  weaker  than  it  was  originally.  So  when  it  is  said  that  wood  has  been  injured  in  the  drying 
process  it  must  be  taken  to  mean  that  it  is  weaker  than  it  should  have  been  after  drying  and 
while  still  in  a  dried  condition. 

When  a  stick  of  timber  dries  out  below  the  fiber-saturation  point  (that  is,  when  it  has 
lost  all  its  free  moisture  and  the  moisture  begins  to  leave  the  cell  walls),  the  timber  begins  to 
shrink  and  change  in  its  mechanical  properties.  Also  numerous  stresses  are  set  up  within  the 
timber.  Under  severe  or  improper  drying  conditions  the  stresses  may  be  great  enough  to 
practically  rum  the  material  for  purposes  where  strength  is  important.  Improper  drying  con- 
ditions, however,  do  not  of  necessity  mean  fast  drying  conditions.  When  properly  dried,  the 
timber  gains  in  its  fiber  stress  at  elastic  limit,  its  modulus  of  rupture,  maximum  crushing  strength, 
etc.  It  bends  farther  at  the  elastic  limit  when  dry  than  when  green,  but  does  not  bend  so  far 
at  the  maximum  load.  After  having  been  bent  to  the  maximum  load  dry  timber  breaks  more 
suddenly  than  green  timber  of  the  same  species— that  is,  dry  timber  is  more  brash  than  green, 
although  it  withstands  greater  stresses  and  is  stiffer. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


11 


DEFECTS  AFFECTING  STRENGTH. 

DIAGONAL   AND    SPIRAL   GRAIN. 


Diagonal  grain  is  produced  when  the  saw  cut  is  not  made  parallel  to  the  direction  of  the 
fibers.  It  can  usually  be  avoided  by  careful  sawing  unless  it  is  caused  by  crooks  in  the  log. 
Spiral  grain,  on  the  other  hand,  results  from  a  spiral  arrangement  of  the  wood  fibers  in  the  tree. 


Fig.  6. — Spiral  grain  in  Sitka  spruce. 

If  a  log  is  spiral  grained,  it  is  impossible  to  secure  straight-grained  material,  except  in  small 
pieces,  from  the  spiral-grained  part.  The  effect  of  spiral  grain  is  illustrated  in  figure  6,  which 
shows  three  views  of  a  piece  of  Sitka  spruce.  The  center  part  of  a  log  may  be  straight  grained 
and  the  outer  part  spiral  grained  or  vice  versa. 


12 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Figures  7  to  14,  inclusive,  show  the  weakening  effect  of  spiral  or  diagonal  grain  upon  various 
strength  properties  of  Sitka  spruce  and  Douglas  fir.  The  data  are  based  upon  about  1,400 
static  bending  tests,  made  upon  clear  specimens,  third  point  loading,  45-inch  span.  Similar 
impact  bending  tests  have  shown  similar  weakening  with  increasing  slope  of  grain. 


7OOO 


/00% 


O/-/ess 


OFGRA/N 


o  Avenage    vaJu.c.s 

•    Probab/e  m/n/~musn 


Fig.  7.—  The  effect  of  spiral  and  diagonal  grain  on  the  fiber  stress  at  the  elastic  limit;  Sitka  spruce. 


- 


-ii 


AIRCRAFT  DESIGN  DATA. 


13 


/oooo 


9OOO 


/.•JO 

SLOPE    Of 


/'/O 


/.O 


mtn/mum 
Fig.  8. — The  effect  of  spiral  and  diagonal  grain  on  the  modulus  of  rupture;  Sitka  spruce. 


14 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


2000 


/<3OO 


/OO% 


o  Menace  va/ues 

?  X77//7//7? 
Fig.  9.— The  effect  of  spiral  and  diagonal  grain  on  the  modulus  of  elasticity;  Sitka  spruce. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


15 


• 


<$ 

3 


8  * 

Nl 
tt 

^* 


t 


/.'4o 
or /ess 


wa*  yes 


7a7, 


^ 


•ea,'? 


\ 


^ 


§3 


80% 
60% 


20% 


/:3o 


/:2o 


\ L L_L 

Fig.  10. — The  effect  of  spiral  and  diagonal  grain  on  the  work  to  maximum  load;  Sitka  spruce, 


\ 


jr 


16 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


/oooo 

9OOO 
&OOO 

7000 

6000 


. 

\ 

>§  S000 


4000 


\ 


3O00 


2000 


/ooo 


/*/: 


/77// 


7/rnu/r? 


era 


X 


•\ 


^ 


\ 


% 


/:4-0  /:30 

or /ess 


SLOPE  OrrGS?/t//V 


/oo% 

(50% 

60% 


0% 


':o 


Fig.  11. — The  eflect  of  spiral  and  diagonal  grain  on  the  fiber  stress  at  the  elastic  limit;  boughs  ..*. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


17 


S2.OOO 


. 
W  L£&EMD 

O 


or/ess 


/77/7S77U/7? 

Fig.  12. — The  effect  of  spiral  and  diagonal  grain  on  the  modulus  of  rupture;  Douglas  fir. 

98257— 19— No.  12 2 


18 


AIECEAFT  DESIGN  DATA. 


Note  12. 


2200 


2OOO 


o/-  /ess 


/:30  /.20 

SLOPE 


607* 


40% 


20% 


0% 


^ 


t/a/ues 

'r?//77u/77  i/a/uesr 


Fig.  13.— The  effect  of  spiral  and  diagonal  grain  on  the  modulus  of  elasticity;  Douglas  fir. 
>n  lo  witoboin  ml)  no  aims  IfinogBib  briH  Iuiiq«  to  } 


Note  12. 


AIECEAFT  DESIGN  DATA. 


19 


The  tests  were  made  upon  seasoned  material,  but  since  the  moisture  content  of  the  indi- 
vidual specimens  varied  somewhat,  it  was  necessary  to  reduce  such  properties  as  are  materially 
affected  by  changes  in  moisture  content  to  a  uniform  basis  before  comparisons  could  be  made. 
Therefore,  the  values  for  fiber  stress  at  the  elastic  limit,  modulus  of  rupture,  and  modulus  of 
elasticity  have  been  reduced  to  11  per  cent  by  means  of  an  empirical  exponential  formula. 
The  work  to  the  maximum  load  values  were  not  reduced  to  a  uniform  moisture  basis,  since  the 
correction  would  have  been  very  small,  and  no  greater  accuracy  would  have  been  insured. 


Fig.  14. — The  effect  of  spiral  and  diagonal  grain  on  the  work  to  maximum  load;  Douglas  fir. 

In  addition  to  the  curve  for  average  values  based  on  test  data,  a  curve  for  probable  mini- 
muni  values  (broken  line)  was  calculated  and  plotted.  A  third  curve  was  also  drawn  showing 
the  probability  of  individual  values  falling  below  the  probable  minimum  value  for  straight- 
grained  material.  This  probability  is  expressed  in  per  cent  and,  as  is  to  be  expected,  increases 
greatly  as  the  slope  of  the  grain  becomes  steeper. 

The  rate  of  falling  off  in  strength  increases  abruptly  at  a  slope  between  1  in  20  and  1  in  15, 
and  therefore  this  slope  may  be  considered  to  be  the  critical  one.  It  is  to  be  noted,  however, 
that  even  at  slopes  at  1  in  20  there  is  a  decided  weakening. 


20 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


As  a  result  of  these  tests  it  is  recommended  that  for  purposes  of  design  the  following  values 
for  moduli  of  rupture  for  spruce  at  15  per  cent  moisture  and  different  slopes  of  spiral  or  diagonal 
grain  be  strictly  adhered  to: 

From  straight  to  1  in  25 7,900  pounds  per  square  inch. 

From  1  in  25  to  1  in  20 7,000  pounds  per  square  inch. 

From  1  in  20  to  1  in  15 5,500  pounds  per  square  inch. 

The  effect  of  spiral  grain  upon  the  maximum  crushing  strength  is  much  smaller  than  upon 
the  modulus  of  rupture.  The  following  stresses  for  different  slopes  of  grain  may  be  used  with 
safety  for  compression  members: 

From  straight  to  1  in  25.. -!••— I I—I— 4,300  pounds  per  square  inch. 

From  1  in  25  to  1  in  20 4,200  pounds  per  square  inch. 

From  1  in  20  to  1  in  15 • 3,800  pounds  per  square  inch. 

When  the  annual  rings  run  diagonally  across  the  end  of  a  piece  the  true  slope  of  diagonal 
grain  can  be  obtained  as  shown  by  figure  15a. 


Slope  of  diagonal 
grain. 


D/L  --  Slope 


of  spiral 
grain. 


Fig.  15. — The  measurement  of  the  slope  of  diagonal  and  spiral  grain. 

The  direction  of  spiral  grain  is  indicated  on  a  tangential  (flat  sawn)  face  by  the  direction 
of  the  resin  ducts.  These  ducts,  however,  are  often  difficult  to  see.  Drops  of  ink  placed  on 
tangential  faces  and  allowed  to  spread  are  sometimes  used  to  test  for  spiral  grain.  The  ink 
will  tend  to  follow  the  angle  of  the  grain.  The  direction  of  spiral  grain  is,  howeVer,  not  given 
correctly  by  resin  ducts  or  by  spreading  of  ink  unless  these  tests  be  applied  to  a  truly  tangen- 
tial face.  In  figure  15,  for  instance,  resin  ducts  or  spreading  of  ink  would  be  practically  parallel 
to  the  edges  whether  the  material  was  spiral  grained  or  not.  In  such  cases  spiral  grain  can  be 
detected  only  by  splitting  on  a  radial  line  (Fig.  156)  or  by  raising  small  splinters  and  observing 
if  they  have  a  tendency  to  tear  deeper  and  deeper. 


Note  12. 


AIRCEAFT  DESIGN  DATA. 


21 


KNOTS. 

The  effect  of  knots  depends  upon  their  location  with  respect  to  the  stresses  to  which  the 
piece  will  be  subjected,  as  well  as  upon  their  size  and  character.  None  but  sound  knots, 
firmly  attached,  should  be  permitted.  Obviously,  knots  of  any  considerable  size  can  not  be 
allowed  in  any  airplane  parts  because  the  parts  themselves  are  comparatively  small  in  cross 
section.  Since  the  weakening  effect  of  knots  results  from  their  disturbance  of  normal  arrange- 
ment of  fibers,  their  seriousness  can  best  be  decided  from  a  consideration  of  the  grain. 

PITCH   POCKETS. 

Tests  recently  completed  on  112  solid  Douglas  fir  wing  beams,  made  especially  to  study 
the  effect  of  pitch  pockets  upon  the  strength  of  beams  indicate  that  this  effect  may  have  been 
overrated  in  previous  specifications.  The  tests  were  made  over  a  72-inch  span  under  third- 
point  loading.  The  following  conclusions  from  these  tests  are  presented  in  the  form  of  speci- 
fications, and  are  intended  to  be  applied  to  spruce  and  fir  wing  beams: 

(a)  In  portions  of  the  length  where  a  slope  of  grain  of  1  in  25  is  the  maximum  allowed, 
pitch  pockets  1|  inches  in  length  and  not  to  exceed  one-eighth  of  an  inch  in  width  or  depth 
may  be  allowed  in  any  portion  of  the  section  except  the  outer  quarters  of  the  flange.  No 
pitch  pockets  to  be  allowed  in  outer  quarters  of  flange. 

(6)  Where  a  slope  of  spiral  grain  of  1  in  20  is  allowed  pitch  pockets  2  inches  in  length 
and  not  to  exceed  one-fourth  inch  in  width  or  depth  may  occur  any  place  in  the  section  except 
in  the  outer  quarters  of  the  flange.  No  pitch  pockets  to  be  allowed  in  outer  quarters  of  flange. 

(c)  Where  a  slope  of  grain  of  1  to  15  is  allowed  pitch  pockets  1J  inches  in  length  and  one- 
fourth  inch  in  width  or  depth  may  occur  in  the  outer  quarters  of  the  flange,  and  pitch  pockets 
3  inches  in  length  and  one-fourth  inch  in  width  or  depth  may  occur  in  any  other  portion  of 
the  section. 

(d)  Pitch  pockets  occurring  in  the  web  may  not  be  closer  together  than  20  inches.     If 
they  are  in  the  same  annual  ring,  they  may  not  be  closer  together  than  40  inches.     In  other 
portions  of  the  section  these  distances  may  be  10  inches  and  20  inches,  respectively. 

Combining  this  specification  with  a  knot  and  spiral-grain  specification,  the  following  table 
has  been  prepared;  it  is  the  intention  that  this  table  be  used  in  drafting  parts  specifications 
for  spruce  and  fir  wing  beams: 

TABLE  1. — Size  and  quantity  of  defects  allowable  witli  different  slopes  of  grain. 


Allowable  slop*  in  grain  not  exceeding  — 

Knots. 

Pitch  pockets. 

Maximum 
diameter 
permitted. 

Minimum 

distance 
l>otween 
any  two. 

Maximum 
length  per- 
mitted. 

Maximum 
width  or 
depth  per- 
mitted. 

1  inch  in  25  

Inches. 
i 

A. 

4 

Inches. 
10 
12 
20 

Inches. 
H 
2 
3 

Inches. 
i 

} 

1  inch  in  20  

1  inch  in  15  

Supplementing  the  table  are  the  following  clauses: 


1.  All  knots  must  be  sound. 

2.  No  defects  must  fall  or  cause  irregular  grain  greater  in  slope  than  that  allowable  for  cross  grain  in  the  outer 
quarter  of  the  upper  or  lower  flange;  except  that  where  a  slope  of  1  in  15  is  allowed,  pitch  pockets  1J  inches  long  ana 
one-fourth  inch  wide  or  deep  may  be  permitted. 

3.  Pitch  pockets  occurring  in  the  web  may  not  be  closer  together  than  20  inches.     If  they  are  in  the  same  annual 
ring,  they  may  not  be  closer  together  than  40  inches.     In  other  portions  of  the  section  these  distances  may  be  10  inches 
aud  20  inches,  respectively. 

4.  The  equivalent  of  the  diameters  specified  may  be  allowed  in  a  number  of  smaller  knots,  provided  that  they 
are  not  close  together. 


22  AIRCEAFT  DESIGN  DATA.  Note  12. 

COMPRESSION   FAILURES   AND   "  CROSS    BREAKS." 

All  material  containing  compression  failures  and  "cross  breaks"  should  be  eliminated 
from  airplane  parts  where  strength  is  of  importance.  The  cause  of  certain  "cross  breaks" 
near  the  center  of  large  logs  such  as  are  quite  frequently  found  in  mahogany  is  not  known. 
Compression  failures,  which  are,  in  fact,  of  the  same  nature  as  "cross  breaks,"  are  known 
frequently  to  be  due  to  injury  by  storm  in  the  standing  trees,  to  carelessness  in  felling  trees 
across  logs,  to  unloading  from  a  car  across  a  single  skid,  or  to  injury  during  manufacture. 

While  some  compression  failures  are  so  pronounced  as  to  be  unmistakable,  others  are 
difficult  to  detect.  They  appear  as  wrinkles  across  the  face  of  the  piece.  Compression  fail- 
ures not  readily  apparent  to  the  eye  may  seriously  reduce  the  bending  strength  of  wood  and 
its  shock-resisting  ability,  complete  failure  occurring  suddenly  along  the  plane  of  injury. 

Figure  16  shows  four  samples  of  African  mahogany  containing  compression  failures  which 
occurred  during  growth.  These  samples  were  later  tested  in  static  bending,  and  in  all  cases 
the  compression  failures  developed  during  te'st  followed  those  originally  occurring  in  the 
samples.  This  is  illustrated  in  figure  17. 

BRASHNESS. 

The  term  "brash,"  frequently  used  interchangeably  with  the  term  "brittle,"  when  used 
to  describe  wood  or  failures  in  wood,  indicates  a  lack  of  toughness.  Brash  wood,  when  tested 
in  bending,  breaks  with  a  short,  sharp  fracture  instead  of  developing  a  splintering  failure  and 
absorbs  a  comparatively  small  amount  of  work  between  the  elastic  limit  and  final  failure.  In 
impact  tests  brash  wood  fails  completely  under  a  comparatively  small  hammer  drop. 

DECAY. 

The  first  effect  of  decay  is  to  reduce  the  shock-resisting  ability  of  the  wood.  This  may 
take  place  to  a  serious  extent  before  the  decay  has  sufficiently  developed  to  affect  the  strength 
under  static  load  or  to  become  evident  on  visual  inspection.  Unfortunately  there  is  no  method 
of  detecting  slight  decay  in  wood  except  with  a  compound  microscope.  AH  stains  and  dis- 
colorations  should  be  regarded  with  suspicion  and  carefully  examined.  It  must  be  remembered 
that  decay  often  spreads  beyond  the  discoloration  it  causes  and  that  pieces  adjacent  to  dis- 
colored areas  may  already  be  infected.  On  the  other  hand,  not  all  stains  and  discolorations 
are  caused  by  decay  of  the  wood.  The  blue  sapstain  of  some  hardwoods  and  of  many  coniferous 
woods,  including  spruce,  and  the  brown  stain  of  sugar  pine  are  not  caused  by  decay-producing 
organisms  and  do  not  weaken  the  wood. 

INTERNAL  OR  INITIAL  STRESSES  IN  WOOD. 

WOOD   FIBERS   UNDER   STRESS   IN   THE   TREE. 

Wood  products  are  quite  similar  to  metal  castings  as  regards  internal  stresses.  It  is 
probable  that  wood  fibers  are  continually  under  stress  of  some  kind.  The  fact  that  freshly 
cut  logs  of  some  species  split  through  the  center  (this  frequently  happens  as  the  result  of  heavy 
shocks  or  jars  and  without  the  use  of  a  wedge)  is  evidence  of  some  tensile  stresses  in  the  outer 
portion  of  the  tree  and  compression  in  the  inner  portion.  These  stresses  are  independent  of 
the  stresses  due  to  the  weight  of  the  tree  and  pressure  against  it. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


23 


Fig.  16. — Compression  failure  occurring  during  growth.     African  mahogany 


24 


AIRCRAFT  DESIGN  DATA.| 


Note  12. 


Fig.  17.— Influence  of  compression  failure  occurring  during  growth  on  failures  in  static  bending.    African  mahogany. 


Note  12.  AIECRAFT  DESIGN  DATA.  25 


INTERNAL    STRESSES   PRODUCED   DURING   DRYING. 

The  natural  stresses  may  be  partially  or  wholly  relieved  by  sawing  the  tree  into  lumber, 
but  other  stresses  are  likely  to  be  introduced  by  subsequent  seasoning.  Checking,  honey- 
combing, warping,  twisting,  etc.,  are  manifestations  of  the  internal  stresses  which  are  produced 
in  the  drying  of  wood  or  whenever  any  change  of  moisture  content  takes  place.  Presumably 
such  stresses  are  due  to  unequal  distribution  of  moisture  and  consequent  unequal  shrinkage 
combined  with  more  or  less  inherent  lack  of  homogeneity. 

Air  drying  for  a  number  of  years,  which  is  practiced  in  some  woodworking  industries,  has 
for  its  object  the  equalization  of  moisture  and  the  relief  of  stresses  induced  in  the  early  part 
of  the  drying.  Careful  and  correct  kiln  drying  followed  by  a  period  of  seasoning  under  proper 
and  controlled  atmospheric  conditions  should  produce  results  at  least  equal  and  probably 
superior  to  those  obtained  by  long  periods  of  air  drying. 

Relieving  these  internal  stresses  is  important  because  they  amount  to  an  actual  weakening 
of  the  material.  If  the  fibers  of  a  piece  of  wood  are  under  stress  when  the  piece  is  free,  they 
are  just  that  much  less  capable  of  resisting  stresses  of  the  same  kind  produced  by  exterior  forces 
or  loads  applied  to  the  piece. 

INITIAL    STRESSES   PRODUCED   IN    ASSEMBLING. 

When  a  member  of  any  structure  is  stressed  in  assembling  the  structure  and  before  any 
load  is  placed  on  it,  it  is  said  to  be  under  initial  stress.  If  the  initial  stress  is  of  the  same  char- 
acter as  the  stress  for  which  the  member  is  designed,  it  constitutes  a  weakening,  for  when  the 
structure  is  loaded  the  safe  working  stress  of  the  member  will  be  reached  just  that  much  sooner. 
If  this  initial  stress  is  opposite  in  character  to  that  for  which  the  member  is  designed,  it  amounts 
to  a  strengthening  of  the  member,  for  when  the  structure  is  loaded  the  initial  stress  must  be 
overcome  before  the  member  takes  any  of  the  stress  for  which  it  is  designed. 

Many  of  the  curved  parts  of  an  airplane  frame  could  be  simply  sprung  to  place  on  assembly. 
Were  this  done,  they  would  be  subjected  to  initial  stress  and  usually  of  the  same  sign  to  which 
the  member  would  later  be  subjected.  In  order  to  avoid  initial  stress,  such  parts  are  steam  bent 
before  assembly.  It  is  desirable,  of  course,  that  this  bending  be  so  done  as  not  to  injure  the 
material  and  to  leave  little  tendency  to  spring  back  from  the  curves  to  which  it  is  bent.  In 
order  that  the  material  may  be  made  sufficiently  plastic  to  accomplish  this  result,  it  is  essential 
that  the  steaming  and  bending  be  carried  out  while  the  wood  is  at  a  relatively  high  moisture 
content.  If  it  is  attempted  on  kiln-dry  or  thoroughly  air-dry  material,  there  is  the  tendency 
to  spring  back  after  the  clamps  are  removed.  Bending  of  such  stock  can  not  be  compared 
to  a  considerable  part  of  the  bending  done  in  other  woodworking  industries,  where  the  strength 
of  the  wood  is  very  greatly  damaged  by  the  bending  process  but  without  destroying  its  use- 
fulness for  the  purpose  for  which  it  is  intended.  Some  of  the  unexpected  failures  of  bent  parts 
in  airplanes  have  doubtless  been  due  to  the  initial  stresses  set  up  in  the  member  during  the 
bending. 

WORKING  STRESSES  FOR  WOOD  IN  AIRCRAFT  CONSTRUCTION. 

Table  2  gives  strength  values  at  15  per  cent  moisture  (which  is  probably  close  to  the  maxi- 
mum moisture  content  of  wood  in  a  humid  atmosphere)  for  use  in  airplane  design,  as  well  as 
the  minimum  specific  gravity  and  average  density  which  should  be  allowed.  It  is  suggested 
that  the  working  stresses  for  design  be  obtained  by  applying  factors  to  the  values  for  static 
load  conditions  as  given  in  this  table. 


26 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


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Note  12.  AIRCRAFT  DESIGN  DATA.  27 

Since  it  is  impractical  to  season  test  specimens  to  precisely  15  per  cent  moisture,  it  was 
necessary  to  compute  the  strength  values  given  in  table  2  at  this  moisture  from  test  data 
obtained  at  slightly  different  moisture  contents.  The  formulae  used  in  these  computations  are 
presented  here  as  a  matter  of  record. 


M  less  than  8,         D15  ='        +B 
M  8  to  10,  P» 


M  10  to  11, 

Mil  to  12,  . 

D1S  =  Strength  at  15  per  cent,  AD  =  air  dry  strength  value,  B  =  green  strength  value,  M  =  per 


cent  of  moisture. 

The  factors  to  be  applied,  and  consequently  the  exact  stress  to  be  used  in  design,  of  course, 
will  depend  largely  on  the  conditions  to  which  it  is  assumed  the  machine  will  be  subjected  in 
flight.  If  they  are  the  most  severe  which  the  machine  is  ever  expected  to  sustain  while  in 
flight,  the  working  stresses  can  be  relatively  high.  If,  on  the  other  hand,  the  assumed  condi- 
tions are  only  moderately  severe,  the  stresses  must  be  made  lower  in  order  to  take  care  of 
exceptional  conditions  which  may  occur.  It  must  also  be  remembered  that  working  stresses 
can  not  be  safely  based  on  average  strength  figures,  but  must  be  lowered  to  a  value  which  will 
be  safe  for  the  weakest  piece  likely  to  be  accepted. 

to  no  boftfed  y\p.  fttftb  ymJnom^jnl 

NATURE   OF   LOADING. 

The  time  of  duration  of  a  stress  on  a  timber  is  a  very  great  factor  in  the  size  of  the  stress 
which  will  cause  failure.  A  continuously  applied  load  greater  in  amount  than  the  fiber  stress 
at  elastic  limit  as  obtained  by  the  ordinary  static  bending  test  will  ultimately  cause  failure. 

The  fiber  stress  at  elastic  limit  in  static  bending  for  the  dry  material  is  usually  somewhat 
more  than  nine-sixteenths  of  the  modulus  of  rupture,  and  in  compression  parallel  to  the  grain 
the  elastic  limit  is  usually  more  than  two-thirds  of  the  maximum  crushing  strength.  Timber 
loaded  slightly  below  the  elastic  limit  will  gradually  give  to  loads  and  ultimately  assume  greater 
deflections  than  those  computed  by  using  the  ordinary  modulus  of  elasticity  figures.  In  impact 
tests  where  a  weight  is  dropped  on  the  stick  and  the  stress  lasts  for  only  a  small  fraction  of  a 
second,  the  stick  is  found  to  bend  practically  twice  as  far  to  the  elastic  limit  as  in  static  tests 
where  the  elastic  limit  is  reached  in  about  two  minutes.  The  elastic  stress  developed  in  the 
stick  undeY  the  blow  is  greater  than  the  maximum  stress  obtained  in  the  static  test. 

TENSILE    STRENGTH. 

In  general  data  on  the  tensile  strength  of  wood  are  little  needed,  and  consequently  there  is 
very  little  data  available.  The  following  table  presents  a  few  figures  on  the  tensile  strength  of 
several  species  tested  green. 

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28 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


TABLE  3. — Strength  of  various  woods  in  tension  parallel  to  grain. 
[From  tests  of  small  clear  specimens  of  green  timber.] 


Species. 

Number 
tests 
averaged. 

Number 
trees 
repre- 
sented. 

Moisture 
content. 

Specific 
gravity. 

Tension  par- 
allel to 
grain  average. 

Probable 
variation  of 
individual 
from 
average. 

Mahogany  African          

20 
27 
0 
50 
59 
63 
48 
10 
7 
4 
42 
5 
13 

5 
7 
4 
18 
9 
10 
10 
10 
2 
2 
9 
3 
5 

Per  cent. 
49.7 
50.1 
47.1 
49.9 
35.0 
24.1 
23.0 
50.0 
39  to  98 
31.0 
41  to  86 
34.5 
40  to  155 

c  0.  457 
c.492 
.550 
c.645 
.399 
c  .530 
c.477 
.  369 
.390 
.401 
.351 
.500 
.400 

Lbs.  per  sg.  in. 
15,  110 
16,  400 
14,  900 
14,  012 
11,  730 
16,200 
13.  300 
7,972 
7,716 
9,  760 
9,580 
9,  8SO 
!),  GOO 

Lbs.  per 
sq.  in. 
2,  075 
2,400 

Mahogany  Central  American  

Oak  northern  white  a                .   

2,900 
1,210 
1,735 
2,  050 
1,400 
1,  570 

Cedar  Port  Orford   

Douglas  fir  (1)       

Douglas  fir  (2)               

Fir  white                        

Hemlock  western                   

Pine  white                                

1,  405 

Redwood               '  

1,170 

a  Not  identified  as  to  species. 

i>  Araucaria  from  Chile,  South  America. 

«  Specific  gravity  based  on  oven-dry  weight  and  volume. 

(1)  Specimens  from  the  8  feet  immediately  above  stump, 
rom  same  trees. 


Other  specific  gravities  based  on  oven-dry  weight  and  volume  as  tested. 

(2)  Specimens  from  the  fifth  8-foot  bolt  above  stump  and  higher.     (1)  and  (2) 


TORSIONAL    STRENGTH. 

• 

Resistance  to  torsion  is  important  in  connection  with  control  surface  spars.     The  following 
fragmentary  data  are  based  on  only  30  tests  in  all,  15  of  each  species: 


TABLE  4. —  Torsional  strength  q/  commercial  white  ash  and  Sitka  spruce. 


Properties. 

• 

White  ash. 

Sitka  spruce. 

Number  of  testa                       

15 
15.8 
.62 
1,  753 
2,  371 
88,  500 
8.8 
24.0 

15 
15.7 
.39 
1,  090 
1,  654 
72,  300 
4.4 
19.7 

Moisture  per  cent  of  oven-dry  weight    

Specific  gravity  (based  on  oven-dry  weight  and  oven-dry  volume)     ..   

Shearing  strength  at  elastic  limit  pounds  per  square  inch      

Shearing  strength  at  maximum  load  pounds  per  square  inch                                               

Shearing  modulus  of  elasticity  pounds  per  square  inch  

Work  to  elastic  limit  inch-pounds  per  cubic  inch  .            .                 

Work  to  first  failure,  inch-pounds  per  cubic  inch  (1)  

(1)  For  the  spruce  and  ash  tested  the  first  failure  oscurred  at  maximum  load  in  all  cases. 

SHRINKAGE. 


Ordinarily  when  a  piece  of  green  lumber  is  dried  no  change  in  dimensions  takes  place  until 
the  fiber  saturation  point  is  reached.  The  wood  then  begins  to  shrink  in  cross-sectional  area 
until  no  further  moisture  can  be  extracted  from  the  cell  walls.  It  also  shrinks  longitudinally, 
but  in  most  cases  the  amount  of  longitudinal  shrinkage  is  so  small  as  to  be  negligible. 

The  shrinkage  in  cross-sectional  area  in  drying  from  the  green  to  the  oven-dried  condition 
varies  with  different  woods,  ranging  from  as  much  as  22  per  cent  (based  on  the  original  area 
before  drying  begins)  to  as  little  as  6  per  cent.  When  dry  wood  absorbs  moisture  it  continues 
to  swell  until  the  fiber  saturation  point  is  reached.  Figures  18,  19,  and  20  illustrate  the  progress 
of  shrinkage  and  swelling  between  zero  moisture  con  tent' and  the  fiber  saturation  point. 


Note  12. 


AIECEAFT  DESIGN  DATA. 


29 


The  shrinkage  of  wood,  like  its  strength,  is  very  closely  related  to  its  specific  gravity. 
This  illustrated  by  figure  21.  On  this  curve,  "Per  cent  shrinkage  in  volume"  is  the  total 
shrinkage  from  fiber  saturation  to  dryness.  It  will  be  noted  that*shrinkage,  in  general,  increases 
with  specific  gravity.  This  relation  in  individual  specimens  of  a  single  species  (white  ash)  is 
shown  in  figure  22. 

Radial  shrinkage,  or  the  shrinkage  in  width  of  quarter  sawn  boards,  averages  about  three- 
fifths  as  great, as  tangential  shrinkage,  or  the  shrinkage  in  width  of  flat  sawn  boards. 


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Fig.  18.  —  Relation  between  swelling  and  moisture.  Each  point  is  the  average  of  from  five  to  eleven  specimens. 
Black  dots  indicate  specimens  that  were  kiln-dried  and  then  allowed  to  reabsorb  moisture.  The  fiber- 
saturation  point  is  at  c. 


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30 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


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MOISTURE  PERCENT 

19.— Relation  between  the  moisture  content  and  the  cross  section  of  small,  clear  pieces  of  western  hemlock. 


K  torn 

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b»r 


ABSORPTION  POHV 


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MOSTURC  PERCCMT 

Fig.  20. — Relation  between  the  moisture  content  and  the  cross  section  of  small,  clear  specimens  of  western  larch. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


31 


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Fig.  21.  —  Relation  between  shrinkage  in  volume  and  specific  gravity  of  various  American  woods. 


.>I-Mif?f 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


List  of  species  and  reference  numbers  for  -figure  21. 
HARDWOODS. 


Species. 

Locality. 

Reference 
No. 

Species. 

Locality. 

Reference 
No. 

Alder,  red  

Washington  

30 

91 
60 
70 
99 
93 
100 
79 
106 
128 
83 
23 
20 
12 
5 
110 
98 

73 
129 
107 
103 
9 
84a 
27 
21 
46b 

72 
24 
46 
40 
6 
59 

151 

125a 
69a 

126 

102 
74 
55 
53 
165 

68 
147 
76 
54 
90 
78 
146 

135 

154 
139 
144 
159 
155 
112 
148 
157 

Hickory  —  Continued  . 
Pignut               

Pennsylvania  

160 
161 
140 
152 
143 
153 
141 
87 
149 
145 

158 
162 
101 

J^.Sa 
66 

58 
69 
92 
56 
104 
108 
124 

125 
80 
163 
121 
133 
116 
130 
137 
119 
118 
117 
97 
94 
142 
150 
115 
111 
132 
138 
136 

131 
109 
122 
105 
164 
35 

85 
51 
156 
49 
89 
61 
63 
65 
45 

11 
-43a 
114 

Ash: 
Biltmore 

Do  

West  Virginia  

Black 

Michigan 

Shagbark  

Mississippi  

Do 

\Viscoiisin 

Do  

Ohio  

Blue 

Xentucky 

Do  

Pennsylvania  

Green 

Louisiana 

Do  

West  Virginia  

Do 

Missouri 

Water  

Mississippi  

Pumpkin  

do           

Holly,  American  

Tennessee  

White 

Hornbeam  

do  

Do 

New  York 

Laurel,  mountain  

do  

Do      

West  Virginia  

Locust: 
Black  

do  

Aspen 

Wisconsin 

Largetooth 

do 

Honey  

Indiana  

Basswood 

Pennsylvania 

Madrona  

California  

Do  
Beech               .... 

Wisconsin  
Indiana               .... 

Do  

Oregon  .... 

Magnolia  

Louisiana  

Do  

Pennsylvania  

Maple: 
Oregon  .  . 

Washington 

Birch: 
Paper  

Wisconsin        

Red  

Pennsylvania  

Sweet      

Pennsylvania 

Do  

Wisconsin    .    . 

Yellow 

do 

Silver  

do 

Do  

Wisconsin      

Sugar  

Indiana  . 

Buckeye,  yellow  

Tennessee       ... 

Do  

Pennsylvania 

Buckthorn,  cascara  .... 
Butternut    

Oregon 

Do  

Wisconsin 

Tennessee  

Oak: 
Bur  

...   .do 

Do            

Wisconsin       

Chinquapin,  western  .  .  . 
Cherry: 
Black  

Oregon  

California  black  
Canyon  live  

California  

Pennsylvania  

do  

Chestnut  

Tennessee 

Wild  red  

Tennessee    

Cow  

Louisiana 

Chestnut  

Maryland  

Laurel  >  

..   ..do  .   . 

Do  

Tennessee  

Post  

Arkansas 

Cotton  wood,  black  

Washington  

Do  

Louisiana 

Cucumber  tree  

Tennessee  

Red  

Arkansas 

Dogwood: 
Flowering  

do  

Do  

Indiana  .... 

Do  

Louisiana 

Western  

Oregon  

Do  

Tennessee 

Elder,  pale  

do  

Highland  Spanish  .  . 
Lowland  Spanish.  .  . 

Louisiana 

Elm: 
Cork  

Wisconsin,     Marathon 
County. 
Wisconsin,    Rusk 
County. 
Indiana  

do  

Swamp  white  

Indiana 

Do  

Tanbark  

California 

Water  

Louisiana 

Slippery... 

White  

Arkansas  ...   . 

Do  

Indiana 

Do  

Wisconsin  

Do 

Louisiana,      Richland 
Parish. 
Louisiana,  Winn  Parish. 
Louisiana 

White  

Pennsylvania 

Do 

Do  

Wisconsin  

Greenheart  

Willow  

Gum: 
Black  

Tennessee  

Yellow  

Arkansas 

Do 

Blue  (Eucalyptus)  . 
Cotton  

California  

Osage  orange  

Indiana 

Louisiana  

Poplar,    yellow    (tulip 
tree). 
Rhododendron  great 

Red    

Missouri 

do 

Hackberry  

Indiana  

Do  

Wisconsin  

Sassafras  

do 

Haw,  pear  

do  

Serviceberry 

do 

Hickory: 
Big  shellbark  

Mississippi  

Silverbell  tree  

do  

Sourwood  

do 

Do  

Ohio  

Sumac  staghorn 

Butternut  

do  

Sycamore 

Mockernut  

Mississippi  

Do  

TGGHGSSGG 

Do  

Pennsylvania  

Umbrella,  Fraser 

do 

Do  

West  Virginia 

Willow: 
Black 

Nutmeg  

Mississippi  

Pignut  

do..  

Do  

Ohio  

Witch  hazel 

Note  12. 


AIRCRAFT  DESIGN  DATA. 


33 


List  of  species  and  reference  numbers  for  figure  21 — Continued. 

CONIFERS. 


Species. 

Locality. 

Reference 
No. 

Species. 

Locality. 

Reference 
No. 

Cedar: 
Incense  

California 

26 
2 
10 
1 
62 
45a 
67a 
46a 

75 
67 
48 

4 
39 
18 
14 
36 
16 
17 

47 
52 
15 
50 
84 
64 

127 
43 
33 
88   ' 
31 
35a 

Pine  —  Continued. 
Lod  ^coole 

Montana,    Granite 
County. 
Montana,        Jefferson 
County. 
Wyoming 

41a 
40a 

34 
123 
113 
96 

95 
57 
71 
86 
77 
22 
82 
42 
19 
37 
41 
32 
25 
28 
13 

8 
3 

44 
29 
7 
38 
81 
134 

Western  red  

Montana 

Do 

Do  

Washington  

White 

Wisconsin 

Do  

Cypress,  bald  

Louisiana  

Douglas  fir  

California  

Longleaf 

Florida 

Do... 

Oregon  

Do 

Louisiana,  Lake  Charles. 
Louisiana,  Tangipahoa 
Parish. 
Mississippi 

Do  

Washington,    Chehalis 
County. 
Washington,     Lewis 
County. 
Washington  and   Ore- 
gon. 
Wyoming  

Do 

Do  

Do 

Do  

Norway  

Wisconsin 

Pitch 

Tennessee 

Do... 

Pond  

Florida 

Shortleaf 

Arkansas 

Fir: 
Alpine  

Colorado  

Sugar  

California  

Table  Mountain  
Western  white 

Tennessee 

Amabilis  

Oregon  

Montana 

Do  

Washington  

Western  yellow  .... 
Do  

Arizona 

Balsam  

Wisconsin  

California 

Grand  

Montana  

Do 

Colorado 

Noble  

Oregon  

Do 

Montana 

White  

California  

White 

\Visconsin 

Hemlock: 
Black  

Montana  

Redwood  

California,  Albion 

Do 

California  Korbel 

Eastern  

Tennessee  

Spruce: 
Engelmann 

Colorado,GrandCounty. 
Colorado,   San  Miguel 
County. 
New  Hampshire 

Do  

Wisconsin  

Western  

Washington  

Do 

Larch,  western  

Montana  

Red 

Do  

Washington  

Pine: 
Cuban  

Florida  

Do  

Tennessee 

White 

New  Hampshire 

Jack  

Wisconsin  

Do 

Wisconsin 

Jeffrey  

California  

Tamarack 

do 

Loblolly  

Florida  , 

Yew  western 

Washington 

Lodgepole  

Colorado 

Do  

Montana,     Gallatin 
County. 

98257— 19— No.  12 3 

e. 


SVO-YTIVAfla 


34 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


SUITABILITY  OF  VARIOUS  AMERICAN  WOODS  FOR  AIRCRAFT  CONSTRUCTION. 

The  difficulty  of  securing  ample  supplies  of  the  woods  heretofore  considered  as  the  standards 
for  aircraft  construction  has  made  it  necessary  to  consider  the  substitution  of  other  species. 
It  must  must  be  realized  that  aircraft  can,  if  necessary,  be  made  from  practically  any  species 
of  wood  which  will  furnish  material  in  the  required  sizes,  and  progress  in  laminating  and  splicing 
has  done  much  to  increase  the  utilization  of  smaller  sized  material.  It  must  also  be  borne  in 
mind  that  the  differences  in  suitability  are  slight  for  a  number  of  species  and  that  high-grade 
stock  of  a  species  considered  to  be  inferior  may  actually  be  better  than  lower  grade  stock  of 


PERCENT  OF  SHRINKAGE  IN  VOLUME 

°>  «  O  N  »  on 

-o 

c 

0 

c 

3 

\ 

\ 

/ 

9 

H' 

/ 

0 

0 

•p 

/ 

WHITE     ASH 

HILATION      OF 

SHRINKAGE  TO 
SPECIFIC   GRAVITY 

PIRCEUT    SMMIMKAai    IS 

TOTAL   men,  0*1111  TO 

OVIN     DRY 

u 

/ 

r 

Di 

U 

/ 

6 

/I 

X) 

/ 

C 

f 

*-d 

S 

/ 

D 

n 

A 

O 

/ 

o 

c 

/ 

3 

/ 

X1 

u 

/ 

/ 

f 

/ 

/ 

/ 

/ 

f 

/ 

/ 

/ 

/ 

y 

f 

2 

/ 

/ 

/ 

y 

/ 

y 

,/ 

/ 

/ 

0  .1  .2  .3  .4  .5  .6  .7  .8  3  1.0 

SPECIFIC    GRAVITY-OVEN    DRY 

•  ASID    OH     OVIH     DRY    VOLUME 

Figure  22. 

the  species  considered  superior.     In  other  words,  it  may  be  preferable  to  change  species  and 
keep  the  grade  up  rather  than  to  lower  the  grade  and  use  the  same  species. 

In  order  to  give  a  general  idea  of  the  relative  properties  of  the  more  common  American 
species  of  timber,  with  respect  to  their  use  in  aircraft,  a  short  statement  concerning  each  has 
been  prepared.  In  those  cases  in  which  the  species  might  possibly  be  considered  as  a  substitute 
for  spruce  its  properties  are  compared  with  those  of  spruce. 

CONIFEROUS    SPECIES. 

Incense  cedar.— This  species  is  somewhat  lighter  than  spruce,  but  lacks  considerably  in 
stiffness  and  does  not  possess  the  toughness  of  spruce.  It  might  be  substituted  for  spruce  for 
parts  which  are  not  highly  stressed. 


Note  12.  AIRCEAFT  DESIGN  DATA.  35 

Port  Orjord  cedar. — Port  Orford  cedar  is  somewhat  heavier  than  Sitka  spruce  and  equals 
or  exceeds  it  in  all  its  strength  properties.  Recent  data  upon  this  species  indicate  that  it  is 
not  as  strong  as  originally  supposed,  but  still  show  it  to  be  equal  to  spruce,  although  of  slightly 
greater  weight. 

Western  red  cedar. — Western  red  cedar  is  lighter  than  spruce  and  below  it  in  all  its  strength 
properties.  It  is  more  difficult  to  dry,  but  could  probably  be  used  with  success  in  many  parts 
where  spruce  is  now  used,  but  could  not  be  used  in  parts  which  are  highly  stressed. 

White  cedar. — White  cedar  is  very  low  in  all  its  strength  properties.  It  is  a  comparatively 
small  tree  and  could  hardly  be  considered  as  a  possibility  for  use  for  the  larger  members. 

Bald  cypress. — Bald  cypress  is  slightly  heavier  than  spruce.  Its  average  figures  show  it 
somewhat  superior  to  spruce  when  used  in  the  same  sizes.  The  great  variability  in  the  wood 
of  this  species  has,  however,  prevented  its  recommendation  for  aircraft  construction.  Cypress 
is  very  wet  in  its  green  condition  and  is  considered  much  more  difficult  to  dry  and  glue  than 
many  other  species. 

Yellow  cypress. — Data  on  this  species  are  not  very  complete.  The  indications  are  that  it 
is  too  low  in  stiffness  to  be  a  satisfactory  substitute  for  spruce. 

Douglas  fir  from  the  Pacific  coast. — Douglas  fir  from  the  Pacific  coast  is  considerably  heavier 
than  spruce  and  all  its  strength  properties  are  equal  to  or  exceed  those  of  spruce.  It  is  quite 
probable  that  the  bulk  of  good  wing-beam  stock  will  come  from  second-cut  logs  and  that  the 
weight  and  corresponding  strength  values  will  run  slightly  lower  than  the  average  of  the  species. 
Douglas  fir  is  considerably  harder  to  dry  than  spruce  and  more  inclined  to  shakes  and  to  check 
during  manufacture  and  to  develop  these  defects  in  service.  It  is  inclined  to  break  in  long 
splinters  and  to  shatter  when  hit.  The  use  of  Douglas  fir  in  the  manufacture  of  wing  beams 
requires  considerably  more  care  than  is  necessary  with  spruce,  but  it  should  give  excellent 
results  (from  the  strength  standpoint)  when  substituted  for  spruce  in  the  same  sizes. 

Douglas  fir,  Rocky  Mountain  type. — The  Rocky  Mountain  type  of  Douglas  fir  is  much 
smaller  than  the  coast  type,  is  quite  knotty  and  somewhat  brash,  and  probably  would  not  be 
satisfactory  as  a  substitute  for  spruce. 

Alpine  fir. — The  Alpine  fir  so  far  tested  was  very  low  in  weight  and  in  all  its  strength  prop- 
erties. This  material  was  from  small  knotty  trees  and  should  not  be  used  except  to  resist  low 
stresses.  It  is  quite  possible  that  the  wood  in  more  extensive  stands  of  comparatively  large 
Alpine  fir  will  be  heavier  and  stronger  than  that  already  tested. 

Amabilis  fir. — The  amabilis  fir  so  far  tested  was  slightly  heavier  than  spruce  and  in  most 
of  its  strength  properties  it  was  practically  the  equal  of  spruce.  Sufficient  data  are  not  at 
hand  to  determine  how  this  material  will  kiln  dry  nor  to  determine  its  working  properties. 
If  it  can  be  kiln  dried  and  worked  satisfactorily,  indications  are  that  it  will  be  a  fairly  satis- 
factory substitute  for  spruce  in  spruce  sizes  in  wing  beams,  struts,  and  other  highly  stressed 
parts. 

Balsam  fir. — Balsam  fir  is  somewhat  lighter  than  spruce  and  considerably  lower  in  all  its 
strength  properties.  It  does  not  give  promise  of  being  satisfactory  in  airplane  construction. 

Grand  fir,  nolle  fir,  and  white  fir. — The  grand  fir  so  far  as  tested  was  slightly  heavier  than 
spruce,  while  the  noble  and  white  fir  were  slightly  lighter.  In  strength  properties  these  species 
compare  very  favorably  with  spruce  except  in  the  case  of  the  shock-resisting  ability  of  white 
fir,  which  is  a  little  low.  This,  however,  may  be  accidental.  The  statement  made  concerning 
amabilis  fir  will  apply  to  these  species  also. 

Black  hemlock. — Black  hemlock  is  quite  a  little  heavier  than  spruce  and  lacking  in  stiffness. 

Eastern  hemlock. — On  a  basis  of  strength  properties  alone  eastern  hemlock  appears  to  be 
a  substitute  for  spruce,  but  the  lumber  is  shaky  and  liable  to  heart  rot,  has  numerous  knots, 
and  develops  shakes  and  checks  in  service.  It  need  not,  therefore,  be  considered. 


36  AIRCRAFT  DESIGN  DATA.  Note  12. 

Western  hemlock. — Western  hemlock  is  heavier  than  spruce,  but  not  quite  so  heavy  as 
Douglas  fir.  It  is  low  in  shock-resisting  ability,  but  on  a  basis  of  strength  alone  it  might  serve 
as  a  substitute  for  spruce  in  spruce  sizes.  No  data  are  available  concerning  proper  kiln-drying 
methods  and  the  possibility  of  manufacturing  conditions  which  would  cause  this  species  to  be 
rejected. 

Western  larch. — Butts  of  the  western  larch  tree  are  very  heavy.  The  material  is  shaky 
and  is  hard  to  dry.  It  would  not  seem  feasible  to  use  this  species  for  aircraft  in  view  of  the 
supply  of  more  suitable  species. 

Cuban  pine. — Cuban  pine  is  entirely  too  heavy  to  be  considered. 

Jack  pine.— The  jack  pine  so  far  tested  was  9  per  cent  heavier  than  spruce  and  was  lacking 
in  stiffness. 

Jeffrey  pine. — Jeffrey  pine  is  especially  lacking  in  stiffness. 

Loblolly  pine. — Loblolly  pine  is  quite  heavy.  It  is  very  variable  in  its  properties  and  need 
not  now  be  considered. 

Lodgepole  pine. — Lodgepole  pine  is  somewhat  low  in  its  shock-resisting  ability  and  slightly 
low  in  stiffness.  If  extensive  stands  of  large  trees  can  be  located,  there  is  a  possibility  that  it 
might  be  found  practicable  to  use  some  of  this  species. 

Longleai  pine. — This  material  is  considered  too  heavy  for  use  in  airplanes  without  redesign. 

Norway  pine. — Indications  are  that  Norway  pine  can  be  used  as  a  substitute  for  spruce  in 
spruce  sizes.  More  data  are  needed  as  to  kiln  drying  and  the  difficulties  which  may  be  met  in 
manufacture. 

Pitch  and  pond  pine. — Pitch  and  pond  pine  are  both  heavy,  and  it  is  not  likely  that  they 
would  ever  be  needed  in  aircraft  work. 

Shortleaf  pine. — The  lighter  material  from  the  shortleaf  pine  could  be  used  for  aircraft 
construction,  but  probably  would  not  be  as  satisfactory  as  Douglas  fir,  since  weight  for  weight 
it  shows  a  lower  modulus  of  rupture  and  stiffness. 

Sugar  pine. — Sugar  pine  is  quite  low  in  shock-resisting  ability  and  stiffness  and  is  quite 
variable.  It  probably  would  not,  therefore,  be  a  suitable  substitute  for  spruce. 

Table  mountain  pine. — Table  mountain  pine  has  about  the  properties  of  shortleaf  pine.  It 
probably  would  not  produce  clear  material  satisfactory  for  aircraft  stock. 

Western  white  pine. — Western  white  pine  is  slightly  heavier  than  spruce  and  shows  up  well 
in  all  its  strength  properties  except  hardness.  It  is  more  difficult  to  dry  than  the  eastern  white 
pine,  but  probably  could  be  substituted  for  spruce  in  spruce  sizes. 

Western  yellow  pine. — Strength  data  show  the  western  yellow  pine  to  be  lacking  in  shock- 
resisting  ability  and  stiffness.  It  is  also  quite  variable.  It  is  not  considered  a  good  substitute 
for  spruce. 

Eastern  white  pine. — Tests  to  date  show  eastern  white  pine  somewhat  below  spruce  in  hard- 
ness and  rather  low  in  shock-resisting  ability.  It,  however,  runs  quite  uniform  in  its  strength 
properties,  is  very  easily  kiln  dried  without  damage,  works  well,  stays  in  place  well,  and  is  rec- 
ommended for  aircraft  construction  as  a  substitute  for  spruce  in  spruce  sizes. 

Redwood. — The  data  available  on  redwood  are  not  comparable  to  those  on  other  species 
and  are  too  erratic  to  form  a  very  definite  judgment  of  the  species.  The  indications  are  that 
the  material  is  quite  variable  in  its  properties  and  likely  to  be  very  brash. 

Engelmann  spruce. — Engelmann  spruce  is  quite  light  and  low  in  all  its  strength  properties. 

Tamarack. — Tamarack  is  too  heavy  to  be  substituted  for  spruce.  It  probably  would  not 
furnish  clear  material. 

Yew. — This  wood  is  very  heavy.     The  tree  is  small  and  crooked. 


Note  12.  AIRCEAFT  DESIGN  DATA.  37 

HARDWOODS. 

Red  alder.— Data  on  this  species  are  very  meager,  but  it  is  probably  not  available  in  sizes 
sufficiently  large  to  make  it  of  importance. 

Biltmore  ash. — Biltmore  ash  should  be  considered  along  with  white  ash  and  may  be  used 
for  longerons  and  other  work  where  strength,  stiffness,  and  ability  to  steam  bend  are  of  impor- 
tance. 

Black  ash. — Black  ash  is  very  low  in  stiffness.  It  is  an  exceedingly  tough  species.  It  is 
one  of  the  best  native  species  for  steam  bending.  It  can  not  be  used,  however,  where  strength 
and  stiffness  are  of  great  importance,  as  in  places  where  white  ash  is  used. 

Blue,  green,  and  white  ash. — These  species  are  known  commercially  as  white  ash  and  are 
very  desirable  for  use  in  longerons  and  other  places  where  steam  bending,  great  strength,  and 
stiffness  are  required. 

Oregon  ash. — Oregon  ash  appears  i,o  be  about  equal  to  the  eastern  white  ash,  although  the 
data  on  this  species  are  somewhat  meager. 

Pumpkin  ash. — Pumpkin  ash  as  a  species  is  somewhat  lighter  than  the  white  ashes.  It  is 
considerably  less  stiff  than  the  white  ash.  Commercially  the  term  is  made  to  include  the  weak, 
soft  material  from  all  the  other  species  of  ash. 

Commercial  white  ash.— Commercial  white  ash  includes  the  Biltmore,  blue,  green,  and  white 
ash  already  mentioned. 

Aspen. — Aspen  is  quite  soft  and  lacking  in  stiffness. 

Basswood. — Basswood  is  light  in  weight  and  low  in  practically  all  its  strength  properties. 
It  is  one  of  the  best  species  to  receive  nails  without  splitting  and  is  used  extensively  for  webs, 
veneer  cores,  and  similar  work. 

Beech. — Beech  is  quite  heavy  and  has  about  the  strength  properties  of  sweet  and  yellow 
birch  and  hard  or  sugar  maple.  It  might  be  used  to  some  extent  in  propellers  but  not  exten- 
sively in  other  aircraft  parts. 

•  Paper  birch. — Paper  birch  is  rather  low  in  its  stiffness  and  high  in  weight. 

Sweet  and  yellow  birch. — Sweet  and  yellow  birch  are  quite  heavy,  hard,  and  stiff.  They 
have  a  uniform  texture  and  take  a  fine  finish.  On  account  of  their  hardness  and  resistance  to 
wear  they  can  be  used  to  face  other  woods  to  protect  them  against  abrasion. 

Yellow  buckeye.- — Yellow  buckeye  is  low  in  its  weight  and  all  its  strength  properties. 

Cascara  buckthorn. — Cascara  buckthorn  is  a  small  tree  and  need  not  be  considered. 

Butternut. — Butternut  is  lacking  in  stiffness  and  probably  need  not  be  considered. 

Western  chinquapin. — Western  chinquapin  is  a  small  tree  and  need  not  be  considered. 

Black  cherry. — Black  cherry  is  a  very  desirable  propeller  wood. 

Wild  cherry.— Wild  cherry  is  a  small  tree  and  lacking  in  stiffness. 

Chestnut. — Chestnut  is  somewhat  heavier  than  spruce  and  is  quite  deficient  in  stiffness. 

Cottonwood. — The  cottonwood  so  far  tested  was  slightly  heavier  than  spruce.  It  is  soft, 
low  in  its  strength  as  a  beam  or  post,  and  lacks  stiffness.  It  is  very  tough,  however,  does  not 
split  hi  nailing,  and  bends  well.  Cottonwood  can  not  well  be  substituted  for  spruce  in  wing 
beams  and  long  struts  but  can  be  used  in  minor  parts. 

Black  cottonwood. — Black  cottonwood  is  low  in  weight  and  all  its  strength  properties. 

Cucumber  tree. — The  wood  of  the  cucumber  tree  is  somewhat  heavier  than  spruce  and  shows 
up  well  in  all  its  strength  properties.  It  is  one  of  the  few  hardwoods  which  gives  promise  of 
being  a  good  substitute  for  spruce  in  wing  beams  and  struts. 

Flowering  and  western  dogwood. — The  dogwood  trees  are  too  small  to  be  considered. 

Elder,  pale. — Elder  is  too  small  a  tree  to  be  considered. 


AIRCRAFT  DESIGN  DATA.  Note  12. 


Elm,  cork  (rock  elm). — Cork  elm  is  slightly  heavier  than  ash.  It  is  low  in  stiffness  and 
very  resistant  to  shocks.  It  steam  bends  well  and  if  properly  dried  can  be  used  for  longerons 
as  a  substitute  for  ash.  Considerably  more  care  is  necessary  in  the  drying  of  elm  in  order  to 
have  it  remain  in  shape  as  it  twists  and  warps  badly  when  not  held  firmly. 

Slippery  elm. — Slippery  elm  is  somewhat  lighter  than  cork  elm,  but  when  of  equal  density 
may  be  used  as  cork  elm. 

White  elm. — Very  dense  pieces  of  white  elm  have  the  requisite  density  and  strength  to  be 
used  along  with  cork  elm.  Most  of  the  white  elm,  however,  is  quite  light.  It  is  lacking  in 
stiffness,  but  steam  bends  well.  It  could  probably  be  used  to  excellent  advantage  in  the  bent 
work  at  the  ends  of  the  wings,  rudders,  elevators,  etc.  Considerable  care  would  be  necessary 
in  order  to  hold  this  material  in  place  while  drying,  as  it  warps  badly. 

Black  gum. — Black  gum  is  considerably  heavier  than  spruce  and  not  nearly  so  stiff.  It 
probably  will  be  but  little  used  in  aircraft. 

Blue  gum  (eucalyptus}. — Eucalyptus  grown  in  this  country  is  quite  heavy.  It  has  large 
internal  stresses,  swells  and  shrinks  excessively,  twists  badly  in  drying,  and  is  very  difficult  to 
dry.  Under  present  conditions  it  probably  should  not  be  used  in  aircraft. 

Cotton  gum  (Tupelo). — This  species  is  considerably  heavier  than  spruce,  but  not  nearly  so 
stiff.  At  present  it  probably  should  not  be  considered  for  aircraft. 

Bed  gum. — Red  gum  is  considerably  heavier  than  spruce  and  superior  to  it  in  strength 
properties.  On  account  of  its  locked  grain  and  its  tendency  to  twist,  warp,  and  check  it  prob- 
ably should  not  be  used  in  place  of  spruce.  There  is  some  prospect,  however,  that  carefully 
quarter-sawed  material  of  this  species  can  be  used  in  propellers. 

Hackberry. — The  denser  pieces  of  hackberry  might  be  substituted  for  ash  in  longerons. 

Pear  haw. — Pear  haw  is  a  very  small  tree  and  of  no  importance  in  this  connection. 

True  hickories,  including  shellbark,  mockernut,  pignut,  and  shagbark. — These  species  are 
heavier  than  ash  and  are  very  tough  and  strong.  They  could  be  substituted  for  ash  in  longerons, 
but  would  probably  not  give  quite  as  good  service  for  the  same  weight. 

Pecan  hickories,  including  butternut,  nutmeg,  pecan,  water. — These  hickories  are  consider- 
ably inferior  to  the  true  hickories,  especially  in  their  ability  to  resist  shock,  and  probably  would 
not  make  satisfactory  substitutes  for  ash. 

American  holly. — This  species  is  lacking  in  stiffness  and  probably  is  of  no  importance  in 
airplane  construction. 

Hornbeam,  California  laurel,  mountain  laurel,  black  locust,  honey  locust,  madrona. — The 
laurels,  locusts,  and  madrona  are  all  heavy  woods  and  probably  have  little  use  in  aircraft 
construction. 

Magnolia. — Magnolia  has  approximately  the  same  properties  as  cucumber  wood,  to  which 
it  is  closely  related,  and  could  probably  be  used  as  a  substitute  for  spruce  in  wing  beams  and 
longerons. 

Oregon  maple. — Oregon  maple  has  about  the  same  properties  as  silver  maple.  It  is  a  little 
more  stiff  and  not  quite  so  resistant  to  shock.  There  is  probably  little  use  for  either  of  these 
species  in  aircraft. 

Red  maple. — Red  maple  is  somewhat  heavier,  stiffer,  and  stronger  than  silver  maple.  Red 
maple  might  possibly  be  used  in  propeller  work,  but  would  give  much  softer  propellers  than 
sugar  maple. 

Sugar  maple.— Sugar  maple  is  quite  heavy,  hard,  and  stiff.  It  could  be  used  with  birch 
in  propeller  manufacture.  It  has  very  uniform  texture  and  takes  a  fine  finish.  On  account 
of  its  hardness  and  resistance  to  wear  it  is  very  often  used  to  face  other  woods  to  protect  them 
against  abrasion. 


Note  12.  AIRCRAFT  DESIGN  DATA.  39 

Silver  maple. — Silver  maple  is  the  lightest  and  softest  of  all  the  maples.  It  is  much  too 
soft  to  be  considered  as  a  substitute  for  sugar  maple  and  lacks  the  stiffness  to  make  it  a  satis- 
factory substitute  for  spruce. 

The  oaks.— The  oaks  need  not  be  considered  as  substitutes  for  spruce,  but  they  play  an 
important  part  in  the  manufacture  of  propellers.  The  oaks  are  all  quite  heavy  and  hard. 
The  oaks,  even  when  a  single  botanical  species  is  considered,  are  extremely  variable  hi  their 
strength  properties.  The  differences  in  the  average  strength  properties  of  the  various  eastern 
oaks  are  not  great,  and  greater  differences  might  readily  be  found  among  different  logs  of  any 
one  species.  The  white  oaks,  as  a  rule,  shrink  and  swell  more  slowly  with  changes  in  the  weather 
than  do  the  red  oaks.  The  radial  shrinkage  of  the  oaks  is  about  one-half  the  tangential  shrinkage. 
This  accounts  for  the  much  greater  value  of  quarter-sawed  oak  over  plain-sawed  oak  for  pro- 
peller construction.  The  southern-grown  oaks  are  much  more  difficult  to  dry  than  are  the 
northern  oaks.  Experiments  are  being  made  in  the  drying  of  both  northern  and  southern 
red  and  white  oaks.  The  northern  white  oaks  when  quarter-sawed  and  carefully  dried  make 
very  satisfactory  propellers.  It  is  possible  that  quarter-sawed  northern  red  oak  will  also  make 
fairly  satisfactory  propellers  but  with  this  disadvantage:  It  is  more  subject  to  defects  in  the 
living  tree,  decays  more  readily,  and  changes  more  rapidly  with  changes  in  weather  conditions. 
To  be  satisfactory  in  this  work  the  southern  oaks  will  require  exceeding  care  in  drying,  as  they 
are  very  difficult  to  dry  without  checking,  honeycombing,  and  casehardening. 

Osage  orange,  persimmon. — Osage  orange  and  persimmon  have  other  very  important  uses 
and  are  probably  of  no  importance  in  aircraft  construction. 

Yellow  poplar. — Yellow  poplar  is  but  little  heavier  than  spruce,  and  while  rather  low  in 
shock-resisting  ability  has  good  working  qualities,  retains  its  shape  well,  is  comparatively  free 
from  checks,  shakes,  and  such  defects.  It  would  probably  be  a  fairly  satisfactory  substitute 
for  spruce  in  wing  beams  and  struts.  It  offers  no  manufacturing  difficulties. 

Rhododendron,  sassafras,  service  berry,  silverbell,  sourwood,  sumac. — These  species  probably 
have  no  place  in  aircraft  construction. 

Sugarberry. — This  species  is  closely  related  to  the  hackberry  and  the  denser  pieces  might 
be  substituted  for  ash  in  longeron  construction. 

Sycamore. — The  trees  are  very  shaky  and  probably  would  not  furnish  material  suitable 
for  aircraft. 

Eraser  umbrella. — This  species  is  closely  related  to  the  cucumber  and  magnolia  previously 
discussed  and  has  similar  properties.  The  clear  stock  obtained  might  be  used  as  a  substitute 
for  spruce. 

Willow,  black  and  western  black,  witch  hazel. — Willow  and  hazel  probably  are  of  no  use  in 
aircraft  construction. 

Walnut,  black. — Black  walnut  has  many  very  important  uses  and  need  not  be  considered 
as  a  substitute  for  spruce.  This  species  probably  makes  the  best  propellers  of  any  of  the 
native  species.  It  is  somewhat  difficult  to  dry,  but  stays  in  place  unusually  well  and  is  hard 
enough  to  resist  wear. 

SYNOPSIS  OF  COMMENTS  AS  TO  SUBSTITUTES  FOR  SPRUCE. 

The  following  species  range  in  weight  from  that  of  spruce  to  25  per  cent  heavier  than  spruce. 
The  data  available  indicate  strongly  that  these  species  can  be  substituted  for  spruce  in  highly 
stressed  parts  using  the  spruce  design:  Port  Orford  cedar,  coast  type  Douglas  fir,  eastern  and 
western  white  pine,  yellow  poplar,  cucumber  tree  and  magnolia.  The  following  species  give 
promise  of  furnishing  substitutes  for  spruce,  but  more  experiments  are  needed  in  order  to  over- 
come known  difficulties  before  these  species  can  be  recommended:  Bald  cypress,  amabilis  fir, 


40 


AIRCRAFT  DESIGN  DATA.  Note  12- 


grand  fir,  noble  fir,  white  fir,  lodgepole  pine,  Norway  pine,  and  redwood.     The  following  spe- 
cies are  lighter  than  spruce,  but  could  be  used  in  parts  where  the  stresses  are  relatively  1 
Incense  cedar,  western  red  cedar,  and  Alpine  fir. 

As  conditions  change  other  species  will  doubtless  come  into  consideration  as  subs 

for  spruce. 

STORAGE  AND  KILN  DRYING   OF  LUMBER. 

The  proper  piling  of  lumber  and  timber  for  air  seasoning  or  as  temporary  storage  previous 
to  kiln  drying  is  extremely  important.  Green  or  partiaUy  dry  stock  is  subject  to  various  forms 
of  deterioration,  such  as  staining,  decay,  severe  checking,  and  (especially  in  hardwoods)  insect 
attack.  During  warm,  humid  weather  staining  may  take  place  in  a  few  days  and  decay  may 
weaken  the  wood  in  a  few  months. 

Proper  piling  of  such  stock  will  tend  to  reduce  the  deterioration  to  a  minimum.  All  lum- 
ber or  timber  which  is  to  be  stored  any  length  of  time  should  be  piled  on  solid  foundations 
with  stickers  between  each  two  courses,  and  should  have  some  protection  from  the  sun  and 
rain.  Whenever  possible,  the  stock  should  be  piled  in  a  shed  with  open  sides.  If  this  is  not 
practicable,  each  pile  should  be  covered  so  as  to  keep  out  rain  and  snow.  Green  hardwoods, 
especially  oak,  frequently  check  severely  at  the  ends.  This  can  be  avoided  to  a  large  extent 
by  coating  the  ends  with  linseed-oil  paint. 

Stock  should  be  cut  up  into  as  small  sizes  as  is  practicable  before  kiln  drying.  Large 
pieces  usually  check  severely  because  the  outer  portion  dries  and  shrinks  considerably  faster 
than  the  inner  core,  which  always  dries  slowly.  Timbers  which  contain  the  pith  and  which 
are  to  be  cut  into  smaller  sizes  later  should  at  least  be  cut  through  the  pith  once,  or,  better, 
be  quartered  before  being  stored  away.  This  will  avoid  the  large  checks  which  are  commonly 
produced  in  the  seasoning  of  timbers  containing  the  pith  by  reason  of  the  tangential  shrinkage 
being  greater  than  the  radial  shrinkage. 

RULES  FOR  PILING  LUMBER. 

1.  The  foundations  should  be  strong,  solid,  and  durable,  preferably  concrete  piers  with 
inverted  rails  or  I  beams  for  skids.     If  this  is  impracticable,  creosoted  or  naturally  durable 
wooden  timbers  should  be  used. 

2.  Each  foundation  should  be  level. 

3.  The  foundations  should  not  be  over  4  feet  apart  for  lumber,  but  may  be  farther  apart 
for  larger  timbers.     For  woods  which  warp  easily  or  for  stock  less  than  1  inch  in  thickness 
foundations  should  not  be  over  3  feet  apart. 

4.  If  the  piles  are  in  the  open,  they  should  have  a  slope  from  front  to  rear  of  1  inch  for 
every  foot  in  length. 

5.  The  foundations  should  be  sufficiently  high  to  allow  the  free  circulation  of  air  under- 
neath the  piles,  and  weeds  or  other  obstructions  to  circulation  should  be  removed. 

6.  Boards  of  equal  length  should  be  piled  together  with  no  free  unsupported  ends. 

7.  A  space  of  about  three-fourths  of  an  inch  should  be  left  between  boards  of  each  layer 
and  from  1  to  2  inches  between  timbers  of  each  layer. 

8.  The  stickers  should  be  of  uniform  thickness,  preferably  seven-eighths  of  an  inch  for  1-inch 
lumber  and  1£  inches  for  thicker  stock. 

9.  Stickers  should  be  placed  immediately  over  the  foundation  beams  and  kept  in  vertical 
alignment  throughout  the  piles.     Their  length  should  be  slightly  in  excess  of  the  width  of  the 
pile. 

10.  The  front  and  rear  stickers  should  be  flush  with  or  protrude  slightly  beyond  the  ends  of 
the  boards. 


Note  12.  AIRCRAFT  DESIGN  DATA. 


KILN  DRYING  OF  WOOD. 

ADVANTAGES   OF   KILN   DRYING. 

The  chief  objects  of  kiln-drying  airplane  stock  are  (a)  to  eliminate  most  of  the  moisture 
in  green  or  partly  dried  stock  more  quickly  than  can  be  done  in  air  drying  and  (&)  to  reduce 
the  moisture  content  of  the  wood  below  that  attained  in  ordinary  air  drying,  so  that  no  more 
drying,  with  consequent  checking,  warping,  and  opening  up  of  seams  will  occur  after  the  wood 
is  in  place.  Other  advantages  incident  to  kiln  drying  are  that  a  smoother  surface  can  be 

obtained  on  kiln-dried  stock  and  that  glues  will  hold  better. 

. 

THE    ELIMINATION   OF   MOISTURE   FROM    WOOD. 

Green  lumber  may  contain  from  about  one-third  to  two  and  one-half  times  its  oven-dry 
weight  of  water.  Expressed  in  percentage,  this  is  from  33  J  to  250  per  cent  moisture  based 
on  the  oven-dry  weight.  The  moisture  content  of  green  lumber  varies  with  the  species,  the 
position  in  the  tree,  whether  heartwood  or  sapwood,  the  locality  in  which  the  tree  grew,  and 
the  drying  which  has  taken  place  since  the  tree  was  cut.  As  a  rule  sapwood  contains  more 
moisture  than  heartwood,  although  in  some  species,  especially  in  butt  logs,  the  heartwood 
contains  as  much  moisture  as  the  sapwood.  Thoroughly  air-dried  lumber  may  contain  from 
about  10  to  20  per  cent  moisture  for  inch  stock  and  more  for  thicker  material. 

Much  of  the  moisture  in  green  wood  is  contained  in  the  cell  cavities  (like  honey  in  a  comb), 
and  the  rest  is  absorbed  by  the  cell  walls.  When  wood  is  drying  the  moisture  first  leaves  the 
cell  cavities  and  travels  along  the  cell  walls  to  the  surface,  where  it  is  evaporated.  When  the 
cell  cavities  are  empty  but  the  cell  walls  are  still  saturated  a  critical  point  is  reached,  known 
as  the  fiber-saturation  point.  Wood  does  not  shrink  or  increase  in  strength  while  seasoning 
until  it  has  dried  below  the  fiber-saturation  point,  which  usually  ranges  between  25  and  30  per 
cent  moisture,  but  may  be  less  or  more,  and  in  spruce  usually  is  between  30  and  35  per  cent. 
This  has  an  important  bearing  on  the  drying  operation,  since  no  casehardening,  checking,  or 
warping  can  occur  so  long  as  the  moisture  content  is  above  the  fiber-saturation  point  in  all  parts 
of  the  stick. 

In  practice  the  stock  should  be  dried  to  a  moisture  content  slightly  less  than  it  will  ulti- 
mately have  when  in  use.  This  may  be  as  low  as  6  per  cent  for  interior  work  and  not  so  low 
for  wood  to  be  exposed  to  weather. 

Two  steps  are  necessary  in  the  drying  of  lumber — (a)  the  evaporation  of  moisture  from  the 
surface,  and  (6)  the  passage  of  moisture  from  the  interior  to  the  surface.  Heat  hastens  both 
these  processes.  For  quick  drying  as  high  a  temperature  should  be  maintained  in  the  kiln  as  the 
wood  will  endure  without  injury.  Some  woods  (especially  coniferous  woods)  will  endure  higher 
temperatures  than  others.  The  general  specifications  for  kiln-drying  airplane  stock  which  follow 
give  the  temperatures  at  which  a  kiln  should  be  operated  to  prevent  injury  to  lumber  to  be  used 
for  airplanes. 

The  lumber  in  a  kiln  is  heated  and  evaporation  is  caused  by  means  of  hot  air  passing  through 
the  piles.  To  insure  proper  drying  throughout  the  piles  a  thorough  circulation  of  air  is  neces- 
sary. The  lumber  must  be  properly  piled  and  the  kiln  constructed  so  as  to  make  the  necessary 
circulation  possible. 

Dry  hot  air  will  evaporate  the  moisture  from  the  surface  more  rapidly  than  it  can  pass 
from  the  interior  to  the  surface,  thus  producing  uneven  drying,  with  consequent  damaging 
results.  To  prevent  excessive  evaporation  and  at  the  same  time  keep  the  lumber  heated  through, 
the  air  circulating  through  the  piles  must  not  be  too  dry;  that  is,  it  must  have  a  certain  humidity. 
The  specifications  give  the  proper  humidities  at  which  to  operate  the  kiln  for  drying  airplane 

,  i 

stock. 

• 


42  AIRCRAFT  DESIGN  DATA.  Note  12. 

THREE    ESSENTIAL   QUALITIES    OF   A    DRY   KILN. 

The  merits  of  any  method  of  drying  airplane  woods  depend  upon  the  extent  to  which  it  affects 
the  mechanical  properties  of  the  stock  and  upon  the  uniformity  of  the  drying.  In  order  that 
complete  retention  of  properties  and  uniform  drying  may  be  guaranteed,  it  is  essential  that  the 
circulation,  temperature,  and  humidity  of  the  air  be  adequately  controlled.  In  this  connection 
circulation  does  not-mean  the  passage  of  air  through  flues,  ducts,  or  chimneys,  but  through  the 
piles  of  lumber,  and  the  terms  temperature  and  humidity  control  apply  to  the  air  within  the 
piles  of  lumber  in  the  kiln. 

Control  of  air  circulation  involves  rate  or  speed  and  uniformity.  A  uniform  passage  of  air 
through  all  portions  of  the  piles  of  lumber  is  the  most  essential  quality  in  a  kiln.  If  the  cir- 
culation can  be  made  both  uniform  and  rapid,  all  portions  of  the  pile  will  dry  quickly  and  at 
the  same  rate.  Furthermore,  uniform  and  rapid  circulation  of  air  are  necessary  before  the 
control  of  temperature  and  humidity  within  the  piles  of  lumber  is  possible. 

When  unsaturated  air  at  any  given  temperature  enters  a  pile  of  lumber  containing  moisture, 
it  exchanges  heat  for  moisture,  is  cooled,  and  rapidly  approaches  saturation.  With  green  wood 
and  a  sluggish  circulation,  the  cooling  is  very  rapid.  The  rate  of  cooling  decreases  as  the  lumber 
dries,  and  if  the  circulation  is  increased  the  loss  of  heat  in  passing  through  the  pile  is  less.  So 
if  the  air  moves  rapidly  through  certain  parts  of  the  piles  and  slowly  through  others,  the  differ- 
ent parts  of  the  piles  will  be  at  different  temperatures.  The  temperature  of  the  air  within  the 
lumber  can  not  be  maintained  at  any  given  value  unless  the  circulation  of  air  is  uniform  at  all 
points  in  the  pile.  Even  though  the  air  moves  at  uniform  speed  from  one  side  of  a  pile  of  lumber 
to  the  other,  if  the  speed  is  too  slow  the  air  loses  its  heat  and  approaches  saturation  rapidly. 
In  general  a  wide  variation  in  the  temperature  of  the  lumber  in  different  parts  of  the  kiln  is 
proof  of  very  uneven  or  slow  circulation.  Inadequate  circulation  and  temperature  control 
render  the  control  of  humidity  and  uniform  drying  impossible. 

Humidity  is  of  prime  importance,  because  the  rate  of  drying  and  the  prevention  of  checking 
and  casehardening  are  directly  dependent  thereon.  It  is  generally  true  that  the  surface  of 
the  wood  should  not  dry  more  rapidly  than  the  moisture  transfuses  from  the  center  to  the 
surface.  The  rate  of  evaporation  must  be  controlled,  and  this  can  be  done  by  means  of  the 
relative  humidity.  Stopping  the  circulation  to  obtain  a  high  humidity  or  increasing  the  circu- 
lation by  opening  ventilators  to  reduce  the  humidity  is  not  good  practice.  Humidity  should 

be  raised,  if  necessary,  to  check  evaporation  without  reducing  the  circulation. 

• 

DEFECTS   DUE   TO   IMPROPER   DRYING. 

Casehardening  and  honeycombing. — When  the  surface  of  a  piece  of  lumber  is  dried  more 
rapidly  than  the  moisture  can  pass  to  it  from  the  ulterior,  unequal  moisture  conditions  exist 
in  the  lumber.  The  moisture  in  the  outer  layers  falls  below  the  fiber  saturation  point.  The 
outer  layers  then  tend  to  shrink  but  are  held  from  shrinking  by  the  more  moist  interior,  which 
has  not  yet  started  to  shrink;  so  the  surface  either  checks  or  dries  in  a  stretched  condition, 
usually  both.  Later,  as  the  interior  dries  it  also  tends  to  shrink  normally,  but  in  turn  is  held 
by  the  outside,  which  has  become  "set"  or  " casehardened."  Consequently,  the  interior  dries 
under  tension,  which  draws  the  outer  layers  together,  closing  up  all  checks  and  producing 
compression.  Casehardened  lumber,  when  resawed,  will  invariably  cup  toward  the  inside  if 
the  interior  if  the  lumber  is  dry  (fig.  23).  If  the  tension  in  the  interior  of  the  wood  is  severe 
enough,  it  may  produce  radial  checks  which  do  not  extend  to  the  surface.  Wood  with  such 
checks  is  said  to  be  honeycombed  or  hollow-horned  (fig.  24).  Casehardening  and  honey- 
combing can  practically  be  prevented  by  regulating  the  humidity  so  that  the  evaporation 
from  the  surface  does  not  take  place  too  rapidly. 


Fig.  23. — Sections  of  casehardened  western  larch  boards.     Nos.  1  and  2  are  original  sections;  Nos.  3  to  8  are  resawed 

sections  showing  cupping;  No.  9  is  one-side  surfaced. 


THE  SAME, 

WELL   KfLN  DRIED  IN 
HUMIDITY  REGULATED 


don 


~__^^^^_^^^_^___^^_. 

' 

-.  -Jloe 

Fig.  24. — Oak  stock  honeycombed  by  air  drying  and  improper  kiln  drying.     Also  similar  stock  properly  dried. 

43 


44 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


If  wood  becomes  casehardened  in  kiln  drying,  it  may  be  brought  back  to  normal  con- 
dition by  steaming,  provided  that  checks  and  cracks  have  not  developed.  Steaming  softens 
the  outer  fibers  and  relieves  the  stresses  caused  by  the  contraction  of  the  outer  shell.  Care 
must  be  taken  not  to  steam  wood  which  has  checked  or  honeycombed  from  casehardening 
enought  to  part  the  fibers  and  weaken  the  piece.  Steaming  will  close  up  the  cracks  but  will 


Fig.  25.— End  view  of  1-inch  boards  of  western  red  cedar  dried  with  and  without  collapse. 

not  restore  the  strength  of  the  piece.     It  will  be  much  harder  to  detect  cracks  and  checks  due 
to  casehardening  if  they  have  been  closed  up  again  by  steaming. 

<Ma^.--Collapse  is  abnormal  shrinkage  causing  grooves  to  appear  in  the  surface  of  the 
lumber  qr*  genaral  distortion  of  the  surface  (fig.  25).  It  is  produced  when  wet  lumber 
is  dried  at  too  high  a  temperature.  The  heat  and  moisture  cause  the  cell  walls  to  'become 
soft  and  plastic.  As  the  water  leaves  the  cell  cavities  the  moist  cell  walls  are  drawn  together 
if  no  air.  is  present.  This  causes  the  cells  to  flatten,  and  a  general  reduction  in  the  cross  sec- 


Note  12.  AIRCRAFT  DESIGN  DATA.  45 

tion  takes  place.  Collapse  occurs  especially  in  such  woods  as  western  red  cedar,  redwood, 
white  oak,  and  others  which  readily  become  soft  and  plastic  when  hot  and  moist.  It  can  be 
avoided  by  not  allowing  the  temperature  to  rise  too  high  while  the  wood  is  still  moist  (at  or 
above  the  fiber  saturation  point). 

Brashness. — High  temperature  treatments  of  all  kinds,  whether  steam  or  hot  air,  are 
injurious  to  lumber,  causing  it  to  turn  darker  and  become  brash.  The  injuries  thus  sustained 
increase  with  the  temperature  and  length  of  time  the  wood  is  exposed  to  such  severe  conditions. 
No  definite  rule  can  be  laid  down  as  to  what  conditions  of  temperature  wood  will  endure  with- 
out becoming  brash.  If  the  temperatures  prescribed  in  the  specifications  (see  p.  68)  are 
not  exceeded,  no  difficulty  will  be  experienced  in  this  respect. 

METHODS   OF   TESTING   CONDITIONS   DURING  DKYING. 

In  drying  airplane  stock  it  is  advisable  to  test  conditions  in  the  kiln  at  frequent  intervals 
so  that  the  operator  will  be  able  to  make  any  changes  promptly  that  the  tests  indicate  are 
necessary  to  maintain  the  proper  rate  of  drying  and  to  prevent  injury  to  the  lumber.  A  con- 
tinuous record  of  proper  conditions  during  kiln  drying  is  a  strong  assurance  of  satisfactory 
stock.  The  following  tests  will  aid  the  inspector  in  keeping  check  on  drying  conditions.  , 

1.  Preliminary  test:  ;  .  , 

(a)  Initial  moisture  conditions  hi  the  lumber. 
(&)  Preparation  and  placing  of  samples. 

(c)  Initial  weights  and  placing  of  whole  pieces. 

(d)  Determination  of  direction,  uniformity,  and  rate  of  air  circulation. 

(e)  Location  and  calibration  of  instruments. 

2.  Current  tests: 

(a)  Determination  of  current  temperatures. 
(6)  Determination  of  current  humidities. 

(c)  Determination  of  circulation. 

(d)  Weighing  of  samples  and  determination  of  current  moisture  conditions. 

3.  Final  tests: 

(a)  Average  kiln-dry  moisture  condition  of  samples. 
(6)  Distribution  of  moisture  in  the  kiln-dry  samples. 

(c)  Determination  of  casehardening  in  kiln-dry  samples. 

(d)  Average  kiln-dry  moisture  condition  of  whole  pieces. 

(e)  Calculation  of  initial  moisture  condition  of  whole  pieces. 
(/)  Distribution  of  moisture  in  kiln-dry  whole  pieces. 

(g)  Distribution  of  casehardening  in  kiln-dry  whole  pieces. 

(h)  Determining  the  effect  of  the  process  on  the  toughness  and  strength  of  the  kiln- 
dry  stock. 

In  making  these  tests  the  following  instruments  and  material  will  be  needed: 
1  sensitive  equal  arm  balance  (capacity,  0.1  to  250  grams). 
1  drying  oven  in  which  the  air  can  be  heated  to  and  held  at  212°  F. 
1  can  of  asphalt  paint  and  a  brush. 
1  sensitive  platform  scale  (capacity,  0.01  to  250  pounds). 

1  electric  flash  light  (lantern  type  recommended). 
12  packages  of  punk  sticks. 

3  accurate  standardized  ordinary  glass  thermometers  (60°  to  230°  F.  by  2°  intervals). 

2  accurate  standardized  glass  wet  and  dry  bulb  hygrometers  with  extra  wicks 
(60°  to  230°  F.  by  2°  intervals). 

Access  to  a  laboratory  equipped  with  machines  for  making  impact,  static  b.ending, 

hardness,  compression  parallel  to  the  grain,  and  other  tests. 
Waxed  or  oiled  paper. 


46  AIRCRAFT  DESIGN  DATA.  Note  12. 

1.  Preliminary  tests.— (a)  Initial  moisture  condition:  Select  at  least  three  representative 
pieces  for  each  10,000  board  feet  of  stock  to  be  dried.     Cut  about  2  feet  from  one  end  of  each. 
Then  cut  a  1-inch  section,  a  24-inch  sample,  and  a  second  1-inch  section  in  succession.     Imme- 
diately weigh  the  two  1-inch  sections  to  an  accuracy  of  one-tenth  of  1  per  cent.     Mark  the 
initial  weights  on  the  section  and  dry  them  to  constant  weight  in  the  oven  heated  to  212°  F. 
Keweigh  them  to  the  same  accuracy  and  determine  the  per  cent  initial  moisture  content  of  thfe 
samples  from  the  formulae : 

Initial  weight— oven-dry  weight 
Per  cent  initial  moisture  content  =  oven-dry  weight 

(6)  Preparation  and  placing  of  samples:  Immediately  after  cutting  the  24-inch  samples 
described  under  (a)  paint  the  ends  of  the  samples  with  a  heavy  coat  of  asphalt  paint.  Then 
weigh  them  separately  on  the  platform  to  an  accuracy  of  one-tenth  of  1  per  cent.  Mark  the 
initial  weights  on  the  samples  and  place  them  in  the  piles  so  as  to  come  under  the  most  severe, 
least  severe,  and  average  drying  conditions,  and  so  as  to  be  subjected  to  the  same  drying  con- 
ditions as  the  adjacent  pieces.  Where  the  circulation  of  air  is  vertical,  place  samples  near  the 
tops,  centers,  and  bottoms  of  the  piles,  and  where  the  circulation  is  lateral  place  them  near 
the  sides  where  the  air  enters  and  leaves  the  piles  and  near  the  centers  of  the  piles. 

(c)  Initial  weights  and  placing  of  whole  pieces:  In  addition  to  the  24-inch  samples  it  is 
desirable  to  select  several  representative  whole  pieces  of  stock  and  weigh  them  to  an  accuracy 
of  one-tenth  of  1  per  cent  on  the  platform  scale.    Mark  the  weights  on  the  pieces  and  place 
them  at  various  points  near  the  tops,  edges,  bottoms,  and  centers  of  the  piles. 

(d)  Determination  of  the  direction,  uniformity,  and  rate  of  air  circulation:  In  order  to 
insure  correct  placing  of  samples,  whole  pieces,  and  instruments  it  is  necessary  that  the  direc- 
tion of  the  circulating  air  be  known.     To  determine  this  light  a  few  punk  sticks,  take  the  flash 
light,  enter  the  kiln,  close  the  door,  and  determine  the  direction,  uniformity,  and  rate  of  motion 
of  the  circulating  air  in  the  spaces  around  the  piles  and  through  the  piles  by  observing  the  smoke 
from  the  burning  punk. 

(e)  Location  and  calibration  of  instruments:  Having  determined  the  direction  in  which 
the  air  passes  through  the  piles,  place  the  bulb  of  the  recording  thermometer  in  contact  with 
a  standardized  glass  thermometer  close  to  the  pile  at  the  center  of  the  side  where  the  air  enters 
the  pile.     If  the  circulation  is  up  through  the  piles,  place  the  thermometer  bulbs  close  under 
the  bottom  center;  if  it  is  down  through  the  lumber,  place  the  bulbs  close  to  the  top  center,  and 
if  the  air  moves  through  the  pile  laterally,  place  the  bulbs  close  to  the  center  of  the  side  where 
the  air  enters  the  pile.     It  is  also  desirable  to  know  the  variation  of  temperature  in  different 
parts  of  the  piles  and  kiln.     To  determine  this  variation,  place  several  of  the  standardized 
thermometers  in  the  tops,  bottoms,  edges,  and  centers  of  the  piles  and  at  different  points  in 
the  kiln.     In  order  to  calibrate  a  recording  thermometer,  place  the  bulb  in  contact  with  a 
standardized  glass  thermometer  in  the  kiln  and  adjust  the  stylus  until  it  agrees  with  the  glass 
thermometer.     The  temperature  must  not  be  fluctuating,  as  is  often  the  case  where  it  is  con- 
trolled by  a  thermostat.     It  is  best  to  use  a  steady  steam  pressure  in  the  heating  pipes  while 
calibrating  instruments.     Never  attempt  to  calibrate  a  recording  thermometer  out  of  its  place 
in  the  kiln. 

To  determine  humidity,  place  the  standardized  glass  wet  and  dry  bulb  hygrometer  near 
the  bulb  of  the  recording  thermometer,  so  as  to  indicate  the  humidity  of  the  air  entering  the 
piles  at  the  tops,  bottoms,  or  edges,  as  the  case  may  be. 

2.  Current  tests. — (a)  Determination  of  current  temperatures:  If  any  part  of  a  pile  is 
exposed  to  direct  radiation  from  the  heating  pipes,  place  a  thermometer  near  the  side  so  exposed. 


Note  12.  AIRCRAFT  DESIGN  DATA.  47 


This  will  indicate  whether  or  not  any  part  is  subject  to  higher  temperature  than  that  indicated 
by  the  recording  instrument.  If  possible,  allow  no  direct  radiation  on  the  lumber.  The  tem- 
perature of  the  air  entering  the  piles  must  be  known  at  all  times,  preferably  by  means  of  recording 
thermometers  with  extension  bulbs  which  have  been  calibrated  in  place,  as  directed  under  1  (e). 

The  temperatures  in  the  tops,  bottoms,  edges,  and  centers  of  the  piles  and  at  different 
points  in  the  kiln  should  be  determined  occasionally  by  using  standardized  thermometers  located 
as  directed  under  1  (e). 

(&)  Determination  of  current  humidities:  Never  attempt  to  determine  the  relative  humidity 
of  the  air  where  the  bulbs  of  the  hygrometer  are  exposed  to  direct  radiation.  Where  direct 
radiation  may  take  place,  it  is  necessary  to  shield  the  hygrometer  from  the  heating  pipes  before 
readings  are  taken.  The  relative  humidity  of  the  air  entering  the  piles  must  be  indicated  at 
all  times  by  means  of  standardized  glass  wet  and  dry  bulb  hygrometers  placed  as  directed  under 
1  (e) .  Before  reading  the  hygrometer  fan  the  bulbs  briskly  for  about  a  minute.  An  air  circu- 
lation of  at  least  15  feet  per  second  past  the  wet  bulb  is  necessary  for  an  accurate  humidity 
reading.  The  wick  should  be  of  thin  silk  or  linen  and  it  must  be  free  from  oil  or  dirt  at  all 
times.  It  should  come  into  close  contact  with  as  much  of  the  bulb  as  possible.  Knowing  the 
correct  wet  and  dry  bulb  hygrometer  readings,  the  relative  humidity  may  be  determined  from 
the  humidity  diagram,  figure  26. 

Relative  humidity  is  shown  on  the  horizontal  scale  and  Fahrenheit  temperature  on  the 
vertical  scale.  The  curves  running  from  the  top  left  to  the  bottom  right  part  of  the  chart  are 
for  various  differences  in  the  wet  and  dry  bulb  readings.  The  curves  are  numbered  near  the 
center  of  the  chart  above  the  heading  "  (t — t1)  degrees  Fahrenheit."  To  get  the  relative  humid- 
ity, follow  the  curve  which  is  numbered  to  correspond  to  the  difference  of  the  wet  and  dry  bulb 
readings  till  it  intersects  the  horizontal  line  numbered  to  correspond  to  the  dry  bulb  reading. 
Directly  below  this  intersection  in  a  vertical  line  will  be  found  the  relative  humidity  on  the 
bottom  scale.  Example:  Dry  bulb  reading,  120;  wet  bulb  reading,  113;  difference,  7.  Curve  7 
intersects  horizontal  line  120  at  vertical  line  79.  Relative  humidity  is  79  per  cent. 

When  the  humidity  is  desired  in  a  Tiemann  kiln,  use  the  set  of  curves  running  from  the 
top  right  to  the  bottom  left  part  of  the  chart.  Locate  the  lower  of  the  two  thermometer  read- 
ings on  the  scale  at  the  right  of  the  chart.  This  is  the  reading  of  the  thermometer  in  the  baffle 
box.  Follow  along  parallel  to  the  nearest  curve  till  the  horizontal  line  is  crossed  whose  number 
is  the  higher  thermometer  reading.  Vertically  below  this  point  of  intersection  on  the  lower 
scale  will  be  found  the  relative  humidity.  Example:  Baffle  thermometer  reading,  112°;  flue 
thermometer  reading,  120°.  Start  at  112  on  right-hand  scale,  follow  parallel  to  curve  28  till 
horizontal  line  120  is  crossed.  This  point  falls  on  vertical  line  80.  Relative  humidity  is  80 
per  cent. 

(c)  Determination  of  circulation:  During  each  drying  operation  the  circulation  of  the  air 
should  be  tested  several  times,  as  under  1  (d) .  As  the  lumber  becomes  drier,  it  has  less  cooling 
effect  on  the  air,  and  this  may  change  the  circulation  in  the  kiln.  If  this  occurs,  correspond- 
ing changes  in  the  location  of  instruments  should  be  made. 

(d}  Weighing  of  samples  and  determination  of  current  moisture  condition:  The  24-inch 
samples,  placed  as  directed  under  1  (6),  should  be  weighed  daily  to  an  accuracy  of  one-tenth 
of  1  per  cent  on  the  platform  scale.  From  test  1  (a)  the  initial  moisture  contents  of  these 
samples  are  known.  Their  initial  weights  were  determined  by  test  1  (6).  Knowing  their 
initial  moisture  contents  and  weights,  their  oven-dry  weights  may  be  computed  from  the 
formula: 

~  .  ,  ,     ,  initial  weight 

Oven-dry  weight  of  sample  =  1AA  .  .   .T.— -, ^r          — T — r  X 100 

'  100  +  initial  moisture  content 


48 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


O'    10    20    30    40    50    60    70     80    90    100 
40  f 


210 


220 


10 


30 


210 


220 

40          50  60  70          80  90  |QO   . 

RELATIVE    HUMIDITY-PER    CENT 

„.     „„ 
Fig.  26. 


Note  12.  AIRCRAFT  DESIGN  DATA.  49 


Having  the  calculated  oven-dry  weights  and  daily  weights  of  the  samples,  their  current 
moisture  contents  may  be  computed  from  the  formula: 

,  current  weight — oven-dry  weight ' 

Current  moisture  content  of  sample  =  -  -T—  -  X  100 

oven-dry  weight 

Therefore,  since  the  samples  were  cut  from  representative  stock,  the  drying  rate  of  the 
material  is  known  currently. 

3.  Final  tests. — -(a)  Average  kiln-dry  moisture  condition  of  samples:  When  the  current 
moisture  contents  of  the  samples  indicate  that  the  material  is  dried  to  the  required  point, 
three  1-inch  sections  are  cut  from  the  center  of  each  sample.  One  section  from  each  sample 
is  used  to  determine  the  average  kiln-dry  moisture  content  of  each  sample  by  the  method  of 
test  1  (a) .  This  test  must  be  made  immediately  after  sawing. 

(6)  Distribution  of  moisture  in  kiln-dry  samples:  A  thin  shell  (about  one-fourth  inch)  is 
split  from  the  four  outer  surfaces  of  the  second  1-inch  section  cut  from  each  sample.  The 
outsides  and  centers  are  tested  for  moisture  content  separately  and  immediately  after  sawing 
by  the  method  of  1  (a).  The  results  of  this  test  show  the  distribution  of  moisture  in  cross 
section  of  the  samples.  The  difference  between  the  moisture  contents  of  the  outer  shells  and 
the  centers  shows  whether  or  not  the  distribution  is  sufficiently  uniform  across  the  sections. 

(c)  Determination  of   casehardening   in  kiln-dry  samples:  The  first  indication  of   case- 
hardening  is  surface  checking.     The  next  sign  of  case-hardening  is  honeycombing  or  interior 
checking  along  the  medullary  rays.     This  defect  can  not  always  be  detected  by  a  superficial 
inspection.     It  is  necessary  to  cut  the  stock  to  discover  it.     Occasionally  it  is  evidenced  by 
a  bulging  of  the  surface  over  the  honeycombed  part.     Often  neither  of  these  defects  is  present. 
In  this  case  the  third  1-inch  section  from  each  sample  is  resawed  two  or  three  times  from  one 
end  down  to  within  about  half  an  inch  of  the  other  end  (see  fig.  23).     If  the  material  is  case- 
hardened  and  dry,  it  will  pinch  the  saw;  if  it  is  not  dry  at  the  time  of  sawing,  the  cupping  of 
the  outer  prongs  will  increase  upon  further  dcying.     If  the  kiln-dried  samples  show  case- 
hardening,  the  material  should  be  steamed  until  the  resawed  sections  do  not  pinch  the  saw  in 
this  test. 

(d)  Average  kiln-dry  moisture  condition  of  the  whole  pieces:  When  the  kiln  is  unloaded, 
the  whole  pieces  from  different  parts  of  the  piles  and  kiln  are  weighed  and  then  cut  as  follows: 
Remove  about  2  feet  from  one  end  and  then  cut  off  three  1-inch  sections.     The  average  kiln- 
dry  moisture  contents  of  the  whole  pieces  are  determined  from  one  section  as  in  test  3  (a). 
The  other  sections  are  used  as  stated  in  3  (/)  and  3  (</). 

(e)  Calculation  of  initial  moisture  condition  of  whole  pieces:  From  the  kiln-dry  weights 
and  kiln-dry  moisture  contents  of  the  whole  pieces,  their  oven-dry  weights  may  be  computed 
from  the  formula: 

Oven-dry  weight  of  whole  pieces  =  ,nn  .  -,  ••,  -:  X  100 

100  +  kiln-dry  moisture  content 

Knowing  the  initial  weights  and  oven-dry  weights  of  the  whole  pieces,  their  initial  moisture 
contents  are  computed  from  the  formula : 

T  ...  ,       -  ,  e     ,    ,  initial  weight — oven-dry  weight 

Initial  moisture  content  of  whole  pieces  =  —  — r—    —  T -r^—  X  100 

oven-dry  weight 

Therefore  the  initial  and  kiln-dry  moisture  conditions  of  the  samples,  whole  pieces,  and 
the  average  stock  are  known. 

(/)  Distribution  of  moisture  in  kiln-dry  whole  pieces:  This  test  is  a  duplicate  of  test 
3(6). 

98257— 19— No.  12 4 


50  AIRCEAFT  DESIGN  DATA.  Note  12. 


(g)  Determination  of  case-hardening  in  kiln-dry  whole  pieces:  This  test  is  a  duplicate  of 
test  3  (c). 

(h)  To  determine  the  effect  of  drying  on  the  strength  of  the  stock:  It  is  practically  impos- 
sible to  determine  the  effect  of  the  process  of  drying  on  the  properties  of  the  stock  by  inspection 
unless  some  visible  defect  has  developed.  This  is  not  usual,  and  as  the  inspector  can  not  always 
resort  to  mechanical  tests  he  should  be  able  to  show  from  his  operation  records  that  conditions 
in  the  kiln  have  been  kept  within  the  specifications  recommended  as  safe  for  kiln-drying  airplane 
stock. 

Detailed  instructions  for  the  kiln  drying  of  various  airplane  woods  have  been  prepared 
and  issued  in  the  form  of  a  specification.  This  specification,  which  follows,  is  based  upon  a 
great  many  experimental  kiln  runs  and  strength  tests  upon  matched  specimens.  Part  of  the 
matched  specimens  were  tested  while  green,  part  were  tested  after  air  drying  under  shelter, 
and  part  were  kiln  dried  to  the  same  degree  as  the  air-dried  specimens  and  then  tested.  In 
this  way  the  effect  of  kiln  drying  as  compared  to  air  drying  was  investigated  and  the  conditions 
of  kiln  drying  were  determined  for  most  rapid  drying  without  decreasing  the  strength  below 
that  obtained  in  air  drying  to  the  same  degree. 

SPECIFICATION    FOR    KILN    DRYING    FOR    AIRCRAFT    STOCK. 


GENERAL. 


1.  This  specification  covers  general  requirements  for  kiln  drying  wood  for  airplane  stock. 

2.  The  kiln-drying  operations  shall  be  so  conducted  that  the  wood  will  not  lose  any  strength, 
toughness,  or  other  physical  property  as  compared  to  wood  air  dried  to  the  same  degree  of 
dry  ness. 


MATERIAL. 


3.  Only  one  species  and  approximately  one  thickness  shall  constitute  a  kiln  charge.     A 
difference  of  not  to  exceed  one-half  inch  in  the  thickness  of  single  pieces  will  be  allowed. 

PILING. 

4.  The  boards  shall  be  piled  so  that  the  horizontal  width  of  the  spaces  between  them  will 
be  at  least  1  inch  for  each  inch  of  board  thickness,  but  in  no  case  shall  the  horizontal  width  of 
such  spaces  exceed  3  inches.     The  boards  must  be  held  flat  and  straight  while  drying. 

5.  For  stock  up  to  four-quarters  (1  inch)  in  thickness  the  crossers  shall  be  at  least  1  inch 
thick  and  not  over  1|  inches  wide. 

6.  For  stock  from  four  to  twelve  quarters  (1  to  3  inches)  in  thickness  the  crossers  shall  be 
at  least  1£  inches  thick  and  not  over  1£  inches  wide. 

7.  For  stock  over  twelve  quarters  (3  inches)  in  thickness  the  thickness  of  the  crossers  shall 
be  increased  in  the  above  proportion  but  must  not  exceed  2  inches  in  any  case. 

8.  The  crossers  shall  be  placed  directly  over  one  another  and  not  over  3  feet  apart  in  the 
courses. 

9.  The  lumber  must  be  so  disposed  in  the  kiln  as  to  permit  of  easy  access  on  both  sides 
of  the  pile  and  the  taking  of  temperature  and  humidity  readings  whenever  required  by  the 
inspector. 

INSTRUMENTS. 

10.  At  least  one  recording  thermometer  or  recording  hygrometer  of  approved  make  shall 
be  used  m  each  dry  kiln  compartment. 

11.  Recording  thermometers  and  hygrometers  shall  be  checked  at  least  once  every  kiln 
run  with  a  standard  thermometer  or  a  glass  thermometer  calibrated  to  an  accuracy  of  1°  F. 
This  comparison  shall  be  made  with  the  thermometers  placed  so  as  to  record  the  maximum 
temperature  of  any  portion  of  the  pile. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


51 


12.  Thermometers. — Thermometer  bulbs  must  be  shielded  from  direct  radiation  from  steam 
pipes,  wet  lumber,  cold  walls  or  surfaces,  and  must  receive  a  free  circulation  of  air. 

13.  The  inspector  may,  at  his  discretion,  place  other  thermometers  at  any  point  in  the  pile. 

14.  Hygrometer. — Humidity  readings  shall  be  made  at  least  three  times  daily  or  more  often 
as  the  inspector  may  desire,  according  to  standard  methods  approved  by  the  inspector,  at  the 
same  points  where  the  bulbs  for  the  recording  thermometers  and  hygrometers  are  placed. 

15.  The  following  shall  constitute  a  standard  method:  Use  a  glass  or  recording  wet  and 
dry  bulb  hygrometer  with  distilled  water  and  with  the  wick  changed  at  least  once  a  week; 
produce  a  circulation  of  air  past  the  wet  bulb  of  at  least  15  feet  per  second  before  reading. 

16.  Hygrometer  bulbs  must  be  shielded  from  direct  radiation  of  steam  pipes,  wet  lumber, 
and  cold  walls  or  surfaces,  and  must  receive  a  free  circulation  of  air. 

STEAMING. 

17.  At  the  beginning  of  the  drying  operations. — Green  wood  is  to  be  steamed  at  a"  tempera- 
ture not  to  exceed  15°  F.  higher  than  the  initial  drying  temperature  specified  in  tables  5  and  6 
for  six  hours  for  each  inch  of  thickness.     Humidity  during  steaming  period  must  be  100  per 
cent,  or  not  below  90  per  cent,  in  every  portion  of  the  pile. 

18.  Previously  air-dried  wood  is  to  be  steamed  at  a  temperature  not  to  exceed  30°  F. 
higher  than  the  initial  drying  temperature  specified  in  tables  5  and  6  for  eight  hours  for  each 
inch  of  thickness.     Humidity  during  steaming  period  must  be  100  per  cent,  or  not  below  90 
per  cent,  in  every  portion  of  the  pile. 

19.  Near  the  end  of  the  drying. — If  on  official  test  the  stock  shows  serious  casehardening 
it  shall  be  steamed  at  a  temperature  not  to  exceed  20°  F.  higher  than  the  final  drying  tempera- 
ture specified  in  tables  5  and  6  for  not  more  than  three  hours.     After  steaming  it  shall  be 
redried. 

TEMPERATURE    AND    HUMIDITY. 

20.  Operating  conditions  are  specified   in  tables  5  and  6,  but  lower  temperatures  and 
higher  humidity  conditions  are  permissible. 

21.  The  progression  from  one  specified  stage  to  the  next  must  proceed  without  abrupt 
changes. 

22.  Green  wood  (above  25  per  cent  moisture}  over  8  inches  thick. — Reduce  the  temperature 
values  given  in  tables  5  and  6  by  5°  F.  for  each  inch  increase  in  thickness. 

23.  Air-seasoned  wood  (below  25  per  cent  moisture)  over  3  inches  thick. — Reduce  the  tem- 
perature values  given  in  tables  5  and  6  by  5°  F.  for  each  inch  increase  in  thickness. 


TABLE  5. 


srll  lo  abii'*  il  iod 


• 
Stage  of  drying. 

Drying  conditions. 

Maximum 
temperature. 

Minimum 
relative 
humidity. 

At  the  beginning  

120 
125 
128 
138 
142 
145 
145 

Per  cent. 
80 
70 
60 
44 
38 
33 
33 

After  fiber  saturation  is  passed  (25  per  cent)  

At  20  per  cent  moisture  

At  15  per  cent  moisture  

At  12  per  cent  moisture  

At  8  per  cent  moisture... 

Final  ... 

.      10 

;te 


• 

lc»iHurtHj'ti»  <*i  italic;  ^\ 


52 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


24.  Table  5  applies  to  the  following  woods: 

Ash,  white,  blue,  and  Biltmore. 
Birch,  yellow. 
Cedar,  incense. 
Cedar,  northern  white. 
Cedar,  western  red. 
Cedar,  Port  Orford. 


Cypress. 

Pine,  sugar. 

Pine,  white  (Idaho  or  eastern), 

Spruce,  eastern  (red  or  white) . 

Spruce,  Sitka. 

Fir,  Douglas. 


TABLE  6. 


Stage  of  drying. 

Drying  conditions. 

Maximum 
temperature. 

Minimum 
relative 
humidity. 

At  the  beginning  

•F 
105 
110 
117 
129 
135 
135 
135 

Per  cent. 
85 
73 
62 
46 
42 
40 
40 

After  fiber  saturation  is  passed  (25  per  cent)  

At  20  per  cent  moisture  

At  15  per  cent  moisture  ;  .  .  .   . 

At  12  per  cent  moisture  

At  8  per  cent  moisture  

Final  

25.  Table  6  applies  to  the  following  woods: 

Cherry. 

Mahogany. 

Oak,  white  and  red. 


Walnut,  black. 
Maple. 


TESTS   DURING    DRYING. 


26.  Samples  shall  be  inserted  in  the  pile  in  such  manner  that  they  will  be  subjected  to  the 
same  drying  conditions  as  that  portion  of  the  pile  where  inserted.     They  shall  be  so  placed 
that  they  can  be  removed  for  periodical  weighing  in  order  to  ascertain  the  average  moisture 
content  of  the  pile  at  any  time. 

27.  Three  samples  shall  be  used  for  each  10,000  board  feet  or  less  of  material  in  the  pile. 
Each  sample  is  to  be  2  feet  long  and  shall  not  be  cut  nearer  than  2  feet  to  the  end  of  one  of  the 
pieces  to  be  dried. 

28.  The  original  moisture  content  of  the  samples  shall  be  determined  from  sections  1  inch 
thick  cut  from  both  ends  of  the  sample  at  the  time  it  is  sawed  from  the  stick.     This  determination 
shall  be  made  as  provided  in  the  specifications.     (See  Appendix,  p.  147.) 

29.  Before  placing  them  in  the  pile,  the  ends  of  the  samples  must  be  given  a  thorough  coating 
of  asphaltum  varnish  to  prevent  end  drying. 

30.  The  samples  shall  be  weighed  to  an  accuracy  of  one-tenth  of  1  per  cent  immediately 
after  cutting  the  moisture  sections  and  before  placing  in  the  kiln.     They  shall  be  weighed  at 
least  daily  when  the  time  of  drying  is  10  days  or  less,  and  at  least  every  other  day  when  the  time 
of  drying  is  more  than  10  days. 

31.  The  samples  shall  be  placed  in  the  pile  and  distributed  so  that  they  will  be  exposed  to 
the  average,  most  rapid,  and  slowest  drying,  except  that  they  shall  not  be  placed  on  the  top 
or  bottom  layers.     The  samples  placed  in  the  portion  of  the  pile  where  drying  is  most  rapid 
shall  control  the  regulation  of  the  temperature  and  humidity. 

32.  After  obtaining  the  dry  weight  of  the  samples,  the  average  moisture  condition  of  the 
pile  during  drying  shall  be  determined  after  each 


Note  12.  AIRCRAFT  DESIGN  DATA.  53 


33.  The  following  example  will  illustrate  the  method  employed: 

Original  weight  of  sample  =  7. 35  pounds. 

Original  moisture  per  cent  (average  of  the  two  1-inch  sections)  =  47. 

Calculated  dry  weight  of  sample=7.35  divided  by  1.47  =  5.00  pounds. 

Current  weight  =  6. 23  pounds. 

Moisture  in  samples  =  6. 23  — 5.00=  1.23  pounds. 

Current  moisture  per  cent  =  (1.23  divided  by  5.00)  X  100  =  24.6. 

34.  Continuous  and  permanent  records  must  be  kept  of  the  temperature  and  humidity 
observations  and  the  percentage  of  moisture  in  the  lumber  in  the  kiln. 


TESTS   AFTER   DRYING. 


35.  Standard  moisture  content  and  case-hardening  tests  shall  be  made  before  the  lumber 
is  removed  from  the  kiln.     Material  for  these  tests  shall  be  taken  from  four  boards  for  each 
5,000  board  feet  or  less  of  material  in  the  pile.     Pieces  selected  must  fairly  represent  the  dried 
stock  and  shall  be  taken  from  different  parts  of  the  pile.     At  his  discretion,  the  inspector  may 
select  other  pieces  for  tests.     Sections  for  these  tests  shall  not  be  cut  nearer  than  2  feet  to  the 
ends  of  the  pieces. 

36.  Three  adjacent  sections  1  inch  thick  shall  be  cut  from  the  centers  of  each  test  piece  of 
stock.     Each  section  must  be  weighed  within  five  minutes  to  prevent  moisture  evaporation. 

37.  The  first  section  (A,  fig.  27)  shall  be  dried  whole  and  the  average  moisture  content 
obtained  as  provided  in  specifications. 

38.  The  second  section  (B,  fig.   27,  moisture  distribution)  shall  be  cut  into   an  outer 
shell  \  inch  wide  and  an  inner  core  £  inch  wide.     The  moisture  content  of  the  outer  shell  and 
inner  core  shall  be  determined. 

39.  The  third  section  (C,  fig.  27)  shall  be  sawed  parallel  to  the  wide  faces  of  the  original 
board  into  tongues  or  prongs,  leaving  about  ^  inch  of  solid  wood  at  one  end  of  the  section. 
For  material  less  than  2  inches  thick  two  saw  cuts  shall  be  made  and  for  material  more  than 
2  inches  thick  five  saw  cuts  shall  be  made.     In  sections  having  six  prongs  the  second  prong 
from  each  side  shall  be  broken  out,  leaving  two  outer  and  two  central  prongs.     The  center 
prong  shall  be  removed  from  sections  having  only  three  prongs. 

40.  The  third  section  shall  then  be  allowed  to  dry  for  24  hours  in  the  drying  room  and 
any  curving  of  the  prongs  noted. 

41.  If  the  prongs  remain  straight,  perfect  conditions  of  stress  and  moisture  content  are 
indicated. 

42.  If  the  outer  prongs  bend  in,  conditions  of  casehardening  are  indicated. 

43.  Only  very  slight  casehardening  is  permissible. 

FINAL   MOISTURE   CONDITIONS. 

44.  An  average  dryness  of  approximately  8  per  cent,  unless  otherwise  specified,*  shall  be 
required.     A  moisture  content  of  from  5  to  11  per  cent  is  permissible  in  individual  sticks. 

45.  The  variation  in  moisture  content  between  the  interior  and  exterior  portions  of  the 
wood,  as  shown  by  the  "moisture  distribution  section"  provided  for  in  paragraph  38,  must 
not  exceed  4  per  cent. 

SEASONING. 

46.  Before  manufacture  the  wood  shall  be  allowed  to  remain  in  a  room,  with  all  parts 
under  uniform  shop  conditions,  at  least  two  weeks  for  3-inch  material  and  other  sizes  in  pro- 
portion.   

*See  Note  3. 


54 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


MOISTURE  &  CtfSE 


TEST 


SECTION ''8" 

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0*5  SPLIT 


TH  tcx  s  rock;  3*rt£p 


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TEST 
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SECTION  tf 


-£7x/.  THICKS  ess 


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J4 


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STOCK 

Fig.  27. — Moisture  and  casehardening  test  specimens. 


Note  12.  AIRCRAFT  DESIGN  DATA.  55 

STEAMING    AND   BENDING    OP   ASH   FOR   LONGERON    CONSTRUCTION. 

47.  The  ash  shall  be  cut  in  the  form  of  rough  squares  sufficiently  large  to  allow  for  shrinkage 
and  finish. 

48.  Where  it  is  necessary  to  bend  this  material,  it  shall  be  steamed  in  the  green  condition 
(more  than  18  per  cent  moisture),  bent  on  forms,  and  then  kiln  dried,  as  provided  in  paragraph  23. 

49.  Steaming  shall  be  conducted  at  a  temperature  not  to  exceed  212°  F.  for  a  period  not 

longer  than  six  hours  and  the  bending  shall  be  accomplished  while  the  material  is  hot. 

* 

INSPECTION. 

50.  At  all  stages  of  the  process  the  lumber  shall  be  subjected  to  inspection  by  the  inspection 
department. 

51.  The  inspector  shall  mark  all  lumber  with  the  official  acceptance  or  rejection  symbol. 

52.  The  inspector  shall  have  free  access  to  every  part  of  the  kiln  at  all  times  and  shall  be 
afforded  every  reasonable  opportunity  to  satisfy  himself  that  this  specification  is  being  complied 
with. 

NOTE  1.  Steaming. — It  has  been  found  possible  to  dry  spruce  satisfactorily  without  steaming  to  relieve  casehard- 
ening.  A  preliminary  steaming  is  given  at  low  temperature,  and  after  the  drying  has  been  completed  the  material 
is  held  in  the  kiln  for  24  hours,  with  a  humidity  of  75  per  cent  or  80  per  cent,  at  room  temperature. 

NOTE  2.  Tests  during  drying. — (Paragraph  31.)  The  most  rapid  drying  sample  should  not  be  confused  with  the 
sample  of  lowest  moisture  content.  If  the  original  moisture  content  was  practically  the  same  for  all  samples,  then  at 
any  stage  of  the  run  the  low  sample  would  be  the  most  rapid  drying.  However,  the  original  moisture  content  is  not 
likely  to  be  uniform  for  the  whole  charge,  and  with  stock  of  varying  moisture  content  the  run  should  be  controlled  for 
the  stock  of  high  moisture  content.  Other  things  being  equal,  the  sample  with  the  highest  moisture  content  will  dry 
the  most  rapidly,  so  that  in  such  a  case  the  specification  would  still  hold.  It  would  therefore  be  desirable  to  place 
the  high  original  sample  where  it  will  be  the  most  rapid  drying  sample.  Otherwise  it  would  be  necessary  to  take  into 
account  the  high  stock — possibly  specify  following  the  average  of  the  samples  on  the  entering  air  side  of  the  pile 
provided  the  average  is  not  more  than  10  per  cent  below  the  high  sample. 

NOTE  3.  Final  moisture  content. — For  naval  aircraft,  it  has  been  found  desirable  to  have  the  moisture  content  on 
removal  from  the  kiln  about  12  per  cent.  The  maximum  individual  variation  allowed  should  not  be  over  3  per  cent. 

TREATMENT    OF   WOOD    AFTER    REMOVAL   FROM    THE    KILN. 

Lumber  should  be  retained  for  at  least  two  weeks  after  removal  from  the  dry  kiln  in  a 
shed  or  room  where  the  conditions  are  approximately  the  same  as  in  the  shop  where  the 
material  is  to  be  worked  up.  The  necessity  for  this  will  be  understood  upon  consideration 
of  the  following  facts:  When  lumber  is  drying  in  the  kiln  the  outer  surface  is  necessarily  some- 
what drier  than  the  interior.  In  good  methods  of  drying  this  difference  is  a  minimum  and 
in  bad  methods  of  drying  it  is  excessive;  but  it  exists  to  a  certain  extent  in  all  methods  of 
drying.  When  the  lumber  has  been  dried  down  to  a  point  somewhat  below  the  condition  to 
which  it  will  finally  come  when  exposed  to  the  normal  shop  working  conditions,  it  will  gradu- 
ally reabsorb  moisture  on  the  outside.  Thus,  thoroughly  kiln-dried  lumber,  if  it  has  stood  in 
an  unheated  room  for  some  time,  will  be  found  to  be  drier  on  the  inside  than  it  is  on  the  sur- 
face, though  the  difference  is  likely  to  be  very  small.  Since  differences  in  moisture  content 
are  indicative  of  internal  stresses  existing  in  the  wood,  it  is  evidently  desirable  to  have  the 
moisture  distribution  as  uniform  as  possible  before  the  lumber  is  made  up  into  finished 
products;  otherwise  the  adjustment  of  stresses,  when  the  lumber  has  been  cut  up,  will  cause 
warping,  checking,  or  other  troubles. 

Just  how  long  lumber  should  remain  in  the  shop  air  after  being  kiln-dried  will  depend, 
of  course,  upon  a  great  many  circumstances.  Generally  speaking,  the  longer  it  remains  the 
better  it  will  be,  provided  the  moisture  conditions  of  the  room  in  which  it  is  stored  are  suitable. 
The  same  kind  of  a  test  as  has  been  explained  for  casehardening  occurring  in  the  dry  kiln  will 
apply  as  a  test  of  the  lumber  after  remaining  hi  storage,  to  see  whether  the  internal  stresses 
have  been  neutralized. 


56  AIRCRAFT  DESIGN  DATA.  Note  12. 

Even  if  casehardening  has  been  removed  in  the  dry  kiln  by  resteaming  at  the  end  of  the 
drying  period,  there  may  still  exist  within  the  lumber  slight  differences  in  moisture  content 
which  will  gradually  adjust  themselves  under  proper  storage  conditions,  so  that  material  which 
has  been  steamed  before  removal  from  the  kiln  is  also  benefited  by  being  allowed  to  stand  in 
the  room  before  it  is  manufactured.  Recent  experiments  have  shown  that  the  length  of  time 
required  for  kiln-dried  stock  to  reach  a  state  of  equilibrium  under  shop  conditions  after  removal 
from  the  kiln  may  be  reduced  very  materially  by  allowing  it  to  remain  in  the  kiln  for  about 
24  hours,  after  the  drying  has  been  completed,  at  a  humidity  of  75  pet  cent  or  80  per  cent  and 
shop  temperature. 

Ideal  conditions  for  the  storage  and  manufacturing  of  lumber  require  regulation  of  the 
humidity,  which  should  be  kept  slightly  below  that  of  the  average  conditions  to  which  the 
lumber  is  to  be  subjected  after  it  is  put  into  service.  The  nearer  these  conditions  are  actually 
met  hi  practice  the  better  are  the  results  to  be  expected,  particularly  where  requirements  are 
so  exacting  as  in  the  construction  of  airplanes. 

CHANGES    OF   MOISTURE    IN   WOOD   WITH   HUMIDITY   OF   AIR. 

Wood  is  a  hygroscopic  material;  that  is,  it  has  the  property  of  absorbing  moisture  from 
the  air  or  surrounding  medium.  It  has  already  been  explained  that  there  are  two  different 
kinds  of  moisture  found  in  wood,  namely,  free  water,  which  occupies  the  openings  in  the  cell 
structure  of  the  wood,  and  hygroscopic  water,  which  is  actually  taken  into  the  cell  waUs  and 
which  upon  being  removed  or  added  to  wood  causes  shrinkage  or  swelling. 

There  is  a  definite  moisture  content  to  which  wood  will  eventually  come  if  it  is  held  in  an 
atmosphere  which  is  at  a  constant  humidity  and  temperature.  The  moisture  content  of  wood 
will  vary  with  the  average  atmospheric  conditions,  also  with  the  size  of  the  material.  Thus, 
ordinary  lumber  which  is  stored  in  the  open  during  the  summer  months  for  sufficient  time 
will  eventually  attain  a  moisture  content  of  from  8  to  15  per  cent,  and  wood  stored  indoors 
in  a  heated  building  will  in  time  fall  to  about  5  or  6  per  cent  because  of  the  lower  relative 
humidity.  If  the  relative  humidity  is  constant,  an  increase  in  temperature  decreases  the 
moisture-holding  power  of  the  wood.  However,  the  moisture  content  is  not  appreciably 
affected  by  temperature  within  a  range  of  25°  to  30°  F. 

Figure  28  shows  the  relation  between  the  moisture  content  of  wood  and  the  humidity 
conditions  of  the  atmosphere.  The  data  for  the  curve  were  obtained  by  keeping  the  wood 
at  a  constant  humidity  and  temperature  until  no  further  change  in  moisture  Occurred.  This 
curve  can  be  used  as  an  aid  in  controlling  the  moisture  conditions  of  wood,  the  approximate 
atmospheric  condition  being  known,  and  in  determining  the  proper  humidities  for  storing 
lumber  hi  order  to  secure  a  certain  moisture  content  and  give  uniform  material  for  use  in  fine 
wood  jointing,  propellers,  etc.  It  is  of  importance  to  have  wood  to  be  used  for  propellers  of 
uniform  moisture  content.  The  curve  may  be  used  also  to  prepare  wood  for  use  in  a  given 
locality,  such  as  the  border  States,  where  the  humidity  is  usually  very  low.  Propellers  for 
use  under  such  conditions  should  be  made  up  at  a  low  moisture  content,  in  order  that  there 
may  be  less  tendency  for  moisture  changes  to  take  place  when  they  are  put  in  service.  It 
must  be  remembered  that  this  curve  must  not  be  used  for  dry- kiln  work  because  of  the  fact 
that  the  dry-kiln  temperatures  used  are  higher  than  those  at  which  the  data  were  collected 
Furthermore,  the  curve  represents  the  ultimate  moisture  content  at  a  given  temperature  and 
humidity,  and  in  the  case  of  large  pieces  of  wood  this  moisture  content  would  not  be  reached 
for  a  long  period  of  time.  Kiln  drying  tends  to  reduce  the  hygroscopic  properties  of  wood, 
hence  curves  for  kiln-dried  wood  are  lower  than  the  one  given.  For  example,  wood  that  had 
been  dried  to  2  per  cent  moisture,  or  less,  if  subjected  to  humidities  between  30  and  70  per 
cent,  would  probably  show  a  corresponding  moisture  content  about  H  to  2$  per  cent  lower 
than  in  the  curve  in  figure  28. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


57 


VENEER  AND  PLYWOOD. 

VENEER. 

Veneer  may  be  loosely  defined  as  thin  wood.  It  usually  varies  in  thickness  from  one- 
hundredth  inch  to  one-eighth  inch,  though  it  is  commercially  possible  to  cut  it  thinner,  and 
thicker  sizes  are  to  be  obtained.  However,  in  general,  veneer  used  in  aircraft  falls  within  the 
limits  stated. 

There  are  three  common  methods  of  manufacturing  veneer,  as  follows:  (1)  The  rotary 
process,  (2)  the  slicing  process,  (3)  the  sawing  process. 

By  far  the  greater  portion  of  all  veneer  manufactured  is  made  by  the  rotary  process. 
Veneer  made  by  this  process  is  all  slash  cut,  and  the  length  along  the  grain  is  limited  by  the 
length  of  the  veneer  lathe.  Rotary  veneer  longer  than  100  inches  is  more  or  less  uncommon. 


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Fig.  28. — Composite  curve  of  moisture  content  of  fine  woods  at  different  humidities  and  ordinary  room  temperature. 

Sliced  veneer  is  usually  manufactured  only  from  the  finer  woods.  On  account  of  the  fact 
that  it  is  possible  to  produce  quartered  veneer  on  slicing  machines,  and  the  waste  on  account 
of  saw  kerf  is  absent,  this  method  of  manufacture  is  preferred  where  pattern  is  important 
and  the  value  of  the  wood  is  great.  The  length  parallel  to  the  grain  of  sliced  veneer  is  limited 
by  the  length  of  the  knife. 

Sawed  veneer  can  be  produced  in  almost  any  reasonable  length  and  from  any  kind  of  stock. 
The  material  produced  may  be  either  quartered  or  slash.  In  general,  sawed  veneer  will  not  be 
specified  for  aircraft  uses,  to  the  exclusion  of  rotary  stock,  except  where  it  is  necessary  to  have 
extra  long  lengths  or  quartered  stock  or  for  some  other  reason  it  is  impossible  to  secure  the 
stock  by  rotary  cutting.  It  may  happen,  for  instance, -that  the  stock  from  which  the  veneer 
is  to  be  cut  can  not  be  handled  to  advantage  in  a  rotary  lathe  on  account  of  its  shape. 


58 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


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Note  12.  AIRCRAFT  DESIGN  DATA.  59 

A  special  series  of  tests  was  made  to  determine  the  effect  of  the  method  of  cutting  veneer 
on  the  strength  of  plywood  panels  made  from  it.  Detailed  results  are  presented  in  table  7, 
and  the  general  conclusions  drawn  follow: 

(a)  The  effect  of  the  method  of  cutting  veneer  on  the  strength  of  plywood  depends  on 
the  species  cut,  although  in  general,  the  effect,  as  shown  by  the  bending  and  tension  tests, 
is  not  great. 

(b)  Of  the  three  methods  of  cutting,  the  sawed  and  sliced  material,  for  the  species  tested, 
gave  the  more  similar  results.     The  commercial  white  ash,  sugar  maple,  and  yellow  poplar 
pannels  cut  by  these  methods  were  slightly  superior  in  bending  and  tensile  strength  to  the 
rotary-cut  panels. 

(c)  For  birch  the  panels  of  rotary-cut  veneer  were  slightly  superior  in  bending  and  tensile 
strength  to  panels  of  either  sawed  or  sliced  veneer. 

(d)  For  the  species  tested,  with  the  possible  exception  of  the  African  mahogany,  panels 
of  sawed  veneer  twist  less  than  panels  of  either  sliced  or  rotary-cut  veneer. 

(e)  With  the  exception  of  birch  the  results  show  little  difference  in  the  twisting  of  panels 
of  sliced  or  rotary-cut  veneer. 

For  the  convenient  calculation  of  the  weight  of  veneer  and  plywood,  table  8  has  been 
prepared.  This  table  presents  the  weights,  per  square  foot,  of  veneer  of  various  thicknesses 
and  species,  at  the  average  air-dry  moisture  condition  shown  in  the  second  column.  The 
weight  of  blood  albumen  glue  per  square  foot  and  the  weight  of  a  typical  casein  glue  (Certus) 
per  square  foot  are  also  given,  so  that  it  is  possible  to  calculate  the  average  weight  of  any  ply- 
wood made  up  of  the  species  listed  and  using  blood  or  casein  glue.  This  is  done  simply  by 
adding  together  the  weights  of  the  individual  plies  and  the  weight  of  the  glue,  which  is  obtained 
by  multiplying  the  weight  of  the  glue  per  square  foot  by  the  number  of  glue  lines  in  the  ply- 
wood. This  number  is  always  one  less  than  the  number  of  plies. 

While  it  is  usually  not  necessary  to  know  the  tensile  strength  of  single-ply  veneer  as  such, 
this  figure  is  very  convenient  in  computing  the  probable  strength  in  tension  of  plywood  made 
up  in  various  manners.  The  last  column  of  table  9  presents  computed  tensile  strengths  of 
single-ply  veneer.  Reference  to  the  other  data  in  this  table  will  be  found  in  the  text  under  the 

discussion  of  plywood. 

PLYWOOD. 

In  general  plywood  consists  of  a  number  of  layers  of  wood  veneer  glued  together  by  some 
suitable  glue  or  adhesive.  Occasionally  the  term  is  applied  to  material  in  which  one  or  more  of 
the  layers  are  composed  of  some  other  material  than  wood. 

The  weight  of  plywood  has  already  been  discussed  in  connection  with  the  weight  of  veneer 
(see  table  8). 

Until  recently  little  information  was  available  on  the  mechanical  properties  of  plywood. 
Within  the  last  year  and  a  half,  however,  about  50,000  tests  have  been  made  and  tabulated. 
Since  the  subject  is  rather  new,  a  full  discussion  is  presented,  followed  by  tables  of  strength 
properties. 

PROPERTIES    OF    WOOD   PARALLEL   AND   PERPENDICULAR   TO   THE   GRAIN. 

Wood,  as  is  well  known,  is  a  nonhomegenous  material  with  widely  different  properties  in 
the  various  directions  relative  to  grain.  This  difference  must  be  recognized  in  all  wood  con- 
struction, and  the  size  and  form  of  parts  and  placement  of  wood  should  be  such  as  to  utilize 
to  the  best  advantage  the  difference  in  properties  along  and  across  the  grain.  It  is  the  strength 
of -the  fibers  in  the  direction  of  the  grain  that  gives  wood  its  relatively  high  modulus  of  rupture, 
and  tensile  and  compressive  strength  parallel  to  the  grain.  Were  it  a  homogenous  material, 
such  as  cast  iron,  having  the  same  strength  properties  in  all  directions  that  it  has  parallel  to 
the  grain,  it  would  be  unexcelled  for  all  structural  parts  where  strength  with  small  weight  is 
desired.  As  it  is  the  tensile  strength  of  wood  may  be  20  times  as  high  parallel  to  the  grain  as 
perpendicular  to  the  grain  and  its  modulus  of  elasticity  from  15  to  20  times  as  high. 


60 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


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Note  12.  AIECEAFT  DESIGN  DATA. 


In  the  case  of  shear  the  strength  is  reversed,  the  shearing  strength  perpendicular  to  the 
grain  being  much  greater  than  the  strength  parallel  to  the  grain.  The  low  parallel-to-the-grain 
shearing  strength  makes  the  utilization  of  the  tensile  strength  of  wood  along  the  grain  difficult 
since  failure  will  usually  occur  through  shear  at  the  fastening  before  the  maximum  tensile 
strength  of  the  member  is  reached. 

The  large  shrinkage  of  wood  across  the  grain  with  changing  moisture  content  may  intro- 
duce distortion  in  a  board  that  decreases  its  uses  where  a  broad  flat  surface  is  desired.  The 
shrinkage  from  the  green  to  the  oven-dry  condition  across  the  grain  for  a  flat  sawn  board  as 
determined  by  the  average  of  150  species  is  about  8  per  cent,  and  for  a  quarter-sawed  board 
about  4£  per  cent,  while  the  shrinkage  parallel  to  the  grain  is  practically  negligible  for  most 
species. 

PLYWOOD   PANELS   V.  SOLID   PANELS. 

It  is  not  always  possible  in  a  given  use  so  to  proportion  a  board  or  solid  panel  as  to  develop 
the  necessary  strength  in  every  direction  and  at  the  same  time  to  utilize  the  full  strength  of 
the  wood  in  all  directions  of  the  grain.  In  such  cases  it  is  the  purpose  of  plywood  to  meet  this 
deficiency  by  crossbanding,  which  results  in  a  redistribution  of  the  material. 

In  building  up  plywood  a  step  is  made  in  obtaining  equality  of  properites  in  two  direc- 
tions— parallel  and  perpendicular  to  the  edge  of  a  board.  The  greater  the  number  of  plies 
used  for  a  given  panel  thickness,  the  more  nearly  homogeneous  in  properties  is  the  finished  panel. 
Thus,  in  an-  airplane  engine  mounting  made  of  15-ply  veneer  the  mechanical  properties  of  the 
panel  in  the  direction  parallel  to  the  grain  of  the  faces  are  almost  the  same  as  those  in  the  direc- 
tion at  right  angles  to  this.  However,  an  increase  in  such  properties  as  bending  strength  and 
modulus  of  elasticity  at  right  angles  to  the  grain  of  the  faces  is  accompanied  by  a  decrease  of 
the  values  parallel  to  the  grain  of  the  faces  with  an  increase  of  the  number  of  plies.  For  a  very 
large  number  of  plies  (of  the  same  species  and  thickness)  we  may  assume  that  the  tensile  strength 
in  the  two  directions  is  the  same  and  that  it  is  equal  to  the  average  of  the  parallel-to-the-grain 
and  perpendicular-to-the-grain  values  of  an  ordinary  solid  board  or  panel.  This  is  not  always 
exactly  true,  since  the  maximum  stress  of  the  plies  with  the  grain  at  right  angles  to  the  force 
may  not  be  reached  at  the  same  time  as  the  maximum  of  the  plies  with  the  grain  parallel  to 
the  force.  Internal  stresses  due  to  change  of  moisture  content  may  also  tend  to  unbalance  the 
strength  ratio. 

SYMMETRICAL    CONSTRUCTION    IN    PLYWOOD. 

On  account  of  the  great  difference  in  shrinkage  of  wood  in  the  direction  parallel  to  the 
grain  and  perpendicular  to  it,  a  change  in  moisture  content  of  plywood  will  inevitably  either 
introduce  or  release  internal  stresses.  Consider,  for  example,  a  three-ply  construction  and 
subject  it  to  low-humidity  conditions,  so  that  the  moisture  content  of  the  plywood  is  lowered. 
Because  the  grain  of  the  core  is  at  right  angles  to  the  grain  of  the  faces,  the  core  will  tend  to 
shrink  a  great  deal  more  than  the  faces  in  the  direction  of  the  grain  of  the  faces.  This  shrinkage 
subjects  the  faces  to  compression  stresses  and  the  core  to  tensile  stresses.  If  the  faces  are  of 
exactly  the  same  thickness  and  of  like  density,  the  stresses  are  symmetrically  distributed  and 
no  cupping  should  ensue. 

Now  consider  that  one  face  of  a  three-ply  panel  has  been  glued  with  the  grain  in  the  same 
direction  as  the  core  and  that  the  moisture  content  of  the  panel  is  reduced.  It  is  obvious  that 
the  internal  stresses  are  now  no  longer  symmetrically  distributed,  inasmuch  as  the  compressive 
stress  in  one  face  has  been  removed.  This  face  now  shrinks  a  great  deal  more  than  the  other 
face  in  the  direction  of  the  grain  of  the  latter.  The  result  is  that  cupping  takes  place.  Figure 
29a  shows  the  effect  of  drying  on  a  three-ply  construction  (unsymmetrical)  in  which  the  grain 
of  two  adjacent  plies  was  parallel.  The  panel  has  curled  up  into  a  cylindrical  surface  with  the 


AIECEAFT  DESIGN  DATA. 


Note  12. 


parallel  plies  on  the  inner  side.  By  adding  another  ply  at  right  angles  to  the  core  we  see  that 
symmetry  could  again  be  established  and  that  while  we  would  have  a  four-ply  panel  in  reality 
it  gives  a  three-ply  construction  with  a  core  of  double  the  face  thickness  and  would  be  regarded 
as  such. 

The  necessity  for  exercising  care  in  sanding  the  faces  of  a  panel  is  obvious,  inasmuch  as 
different  thicknesses  on  the  faces  would  introduce  unequal  forces  with  changing  moisture  content. 

In  order  to  obtain  symmetry,  it  is  also  necessary  that  both  faces  or  symmetrical  plies  be 
of  the  same  species. 

To  summarize:  A  veneer  panel  must  be  symmetrically  constructed  in  order  to  retain  its 
form  with  changes  of  moisture.  Symmetry  is  obtained  by  using  an  odd  number  of  plies.  The 


Fig.  29.— (a)  Cupping  resulting  from]  unsymmetetrical  construction  in  plywood,     (b)  Twisting  resulting  from  ply- 
wood construction  with  grain  of  faces  at  45  degrees  with  grain  of  core. 

plies  should  be  so  arranged  that  for  any  ply  of  a  particular  thickness  there  is  a  parallel  ply  of 
the  same  thickness  and  of  the  same  species  on  the  opposite  side  of  the  core  and  equally  removed 
from  the  core. 

DIRECTION    OF   THE    GRAIN    OF    ADJOINING    PLIES. 

In  the  discussion  of  symmetry  of  construction  it  was  understood  that  the  adjoining  plies 
were  always  glued  with  the  grain  either  parallel  to  or  exactly  at  right  angles  to  the  core.  In  care- 
less construction  this  may  not  always  be  the  case.  An  extreme  case  of  this  kind  is  shown  in  figure 
29b,  in  which  the  plies  were  glued  so  that  the  grain  of  each  face  of  the  panel  was  at  45  degrees 
with  the  grain  of  the  core  and  so  that  the  two  faces  were  at  90  degrees  with  respect  to  each  other. 
Whereas  the  unsymmetrical  construction  introduces  cupping,  a  construction  involving  angles 
other  than  0  and  90  degrees  introduces  twisting. 

In  building  up  a  three-ply  veneer  panel  the  core  should  be  glued  with  the  grain  at  90 
degrees  with  the  faces  or  as  close  to  this  as  feasible. 

EFFECT   OF   MOISTURE    CONTENT. 

. 

The  previous  discussion  has  brought  out  the  fact  that  a  change  in  moisture  content  of  a 
panel  may  introduce  cupping  and  twisting  in  the  panel  if  the  panel  is  not  carefully  constructed. 
Hence  it  is  highly  desirable  that  the  moisture  content  of  the  veneer  before  gluing  be  controlled 


Note  12.  AIRCRAFT  DESIGN  DATA.  63 


so  as  to  make  the  moisture  content  of  the  finished  panel  when  it  leaves  the  clamps  about  the 
same  as  it  will  average  when  in  use  and  that  all  plies  be  at  the  same  moisture  content  before 
gluing.  The  limits  of  from  10  to  15  per  cent  moisture  in  the  finished  panel  will  usually  give 
satisfactory  results  when  the  panel  is  in  service  in  the  open  air. 

SHRINKAGE    OF    PLYWOOD. 

The  shrinkage  of  plywood  will  vary  with  the  species,  the  ratio  of  ply  thickness,  the  number 
of  plies,  and  the  combination  of  species.  The  average  shrinkage  obtained  in  54  tests  on  a  variety 
of  combinations  of  species  and  thicknesses  in  bringing  three-ply  wood  from  the  soaked  to  the 
oven-dry  condition  was  0.45  per  cent  parallel  to  the  face  grain  and  0.67  per  cent  perpendicular 
to  the  face  grain,  with  the  ranges  of  from  0.2  to  1  per  cent  and  0.3  to  1.2  per  cent,  respectively. 
Other  combinations  and  thicknesses  may  extend  these  limits  and  change  the  average  somewhat. 
The  species  included  in  the  tests  made  were  mahogany,  birch,  poplar,  basswood,  red  gum, 
chestnut,  cotton  gum,  elm,  and  pine. 

EFFECT   OF   VARYING    THE    NUMBER    OF    PLIES. 

The  question  frequently  arises,  Should  three  or  more  plies  be  used  for  a  panel  of  a  given 
thickness?  The  particular  use  to  which  the  panel  is  to  be  put  must  answer  this  question. 
Commercial  considerations  will  also  enter.  Veneer  of  most  species  less  than  7V  mcn  thick  can 
not  be  cut  by  the  rotary  process  with  uniform  success,  and  while  a  number  of  species  may  be 
cut  by  slicing  to  -fa  inch  and  less,  such  material  is  limited  in  width. 

In  general  it  may  be  said  that  the  greater  the  number  of  plies  the  flatter  the  plywood  will 
remain  when  subjected  to  moisture  variations. 

If  the  same  bending  or  tensile  strength  is  desired  in  the  two  directions,  parallel  and  per- 
pendicular to  the  grain  of  the  faces,  the  greater  the  number  of  plies  the  more  nearly  the  desired 
result  is  obtained.  This  same  result  may  be  obtained  by  a  proper  selection  of  ratio  of  core  to 
total  plywood  thickness  in  three-ply  construction.  It  must  be  borne  in  mind,  however,  that 
a  plywood  with  a  large  number  of  plies,  while  stronger  at  right  angles  to  the  grain  of  the  faces, 
can  not  be  as  strong  parallel  to  the  grain  of  the  faces  as  three-ply  wood,  and  hence  a  three-ply 
panel  is  preferable  where  greater  strength  is  desired  in  one  direction  than  in  the  other.  Table 
1 1  gives  strength  values  for  three-ply,  five-ply,  and  seven-ply  yellow  birch  plywood. 

Where  great  resistance  to  splitting  is  desired,  such  as  in  plywood  that  is  fastened  along 
the  edges  with  screws  and  bolts  and  is  subject  to  forces  through  the  fastenings,  a  large  number 
of  plies  affords  a  better  fastening. 

It  is  a  common  experience  that  a  glued  joint  is  weakened  when  two  heavy  laminations  are 
glued  with  the  grain  crossed.  The  same  weakness  exists  in  plywood  when  thick  plies  are 
glued  together.  When  plywood  is  subject  to  moisture  changes,  stresses  in  the  glued  joint 
due  to  shrinkage  are  greater  for  the  thick  plies  than  for  the  thin  plies.  Hence  in  plywood 
constructed  with  many  thin  plies  the  glued  joints  will  not  be  as  likely  to  fail  as  in  plywood 
constructed  of  a  smaller  number  of  thick  plies. 

EFFECT  OF  VARYING  THE  RATIO  OF  CORE  TO  TOTAL  THICKNESS. 

At  first  thought  it  may  seem  that  the  proper  selection  of  the  ratio  of  core  to  total  plywood 
thickness  in  three-ply  construction  may  enable  the  designer  to  get  the  same  strength  in  both 
directions  as  is  possible  with  many-plied  panels.  While  this  is  true  in  general,  it  is  not  true 
that  the  same  ratio  will  serve  for  both  tension  and  bending.  In  birch,  for  example,  a  ratio  of 
core  to  total  plywood  thickness  of  5  to  10  gives  the  same  strength  in  tension  in  both  directions, 


AIRCRAFT  DESIGN  DATA.  Note  12. 


but  a  ratio  of  about  7  to  10  gives  the  same  strength  in  bending.  For  either  ratio  the  plywood 
is  not 'nearly  as  resistant  to  splitting  as  plywood  of  a  greater  number  of  plies  totaling  the  same 
thickness. 

SPECIES    OF   LOW   DENSITY   FOR    CORES. 

Where  column  strength  and  a  flat  panel  are  desired,  full  advantage  of  a  strong  species, 
such  as  birch,  in  the  faces  is  best  attained  by  using  a  thick  core  of  a  species,  such  as  basswood 
or  yellow  poplar,  rather  than  a  thinner  core  of  the  same  weight  but  of  a  species  of  geater  density. 
A  combination  of  strong  faces  and  a  thick  light  wood  core  has  the  advantage  of  greater  separa- 
tion of  the  faces  than  when  using  the  thinner  core  of  a  heavier  species,  giving  a  marked  increase 
in  the  internal  resistance  to  forces  that  tend  to  bend  the  panel  and  a  correspondingly  great 
strength  in  bending  with  the  same  weight. 

Consider,  for  example,  that  a  certain  panel  contains  a  core  of  the  same  weight  but  of  a 
specific  gravity  of  one-half  that  of  another  core.  This  means  that  the  core  of  lighter  species 
is  twice  as  thick  as  the  core  of  high  density  and  that  the  panel  faces  are  spaced  twice  as  far 
apart.  In  a  long  column,  for  instance,  this  is  very  desirable,  for  the  maximum  load  a  column 
can  carry  varies  as  the  cube  of  the  thickness.  It  is  evident  that  a  marked  superiority  in  the 
load  sustained  might  be  expected  in  the  low-density  core  panel  over  the  high-density  core  panel 
of  the  same  weight  when  the  load  is  applied  parallel  to  the  grain  of  the  faces. 

The  same  line  of  reasoning  applied  to  column  strength  may  also  be  applied  to  resistance 
to  cupping.  A  panel  with  a  core  of  low  density  will  cup  less  than  a  panel  of  the  same  weight 
with  a  core  of  high  density.  The  load  to  produce  failure  in  bending  would  likewise  be  greater 
for  the  former  case. 

PLYWOOD   TEST   DATA. 

The  column-bending  modulus  is  obtained  by  loading  a  piece  of  plywood  5  inches  by  12 
inches  as  a  column  with  the  12-inch  length  vertical.  It  is  computed  by  the  following  formula: 

P     6M 

S=A+5J2'  where 

S  =  Column-bending  modulus. 

A  A  t 

A  =  Area  of  cross  section. 

P  ==  Load  at  maximum  moment. 

. 
M  =  Maximum  bending  moment. 

T  TTT-    1     1  t  • 

o  =  Width  of  test  piece. 
d  =  Thickness  of  test  piece. 

Like  the  modulus  of  rupture  in  the  standard  static  bending  test,  the  column-bending 
modulus  is  not  a  true  stress  existing  in  the  fibers  at  the  instant  of  failure.  It  is  merely  a  measure 
of  the  magnitude  of  the  external  bending  moment  that  a  piece  of  plywood  can  withstand  before 
it  fails. 

If  a  piece  of  plywood  is  subjected  to  forces  that  tend  to  bend  it,  as  would  be  the  case 
either  in  a  long  column  or  in  a  beam,  the  designer  confronted  with  the  problem  of  determining 
its  proper  thickness  may  use  the  column-bending  modulus  in  exactly  the  same  way  that  the 
modulus  of  rupture  is  used.  It  will  be  noted,  of  course,  that  the  column-bending  modulus 
must  be  used  which  applies  to  the  particular  plywood  construction  desired.  The  total  plywood 
thickness  is  to  be  used  in  all  equations  involving  the  column-bending  modulus. 

The  use  of  the  tensile  strength  data  is  obvious.  The  strength  values  given  are  based  on 
the  total  plywood  thickness,  (Table  9.) 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


65 


TABLE  No.  9. —  Tensile  strength  of  plywood  and  veneer. 


Number 
of  tests. 


Moisture  at 

test 
(per  cent). 


Specific 
gravity  * 
of  ply- 
wood. 


Tensile 

strength  f  of 

3-ply  wood 

parallel  to 

grain  of  faces 

(pounds  per 

square  inch). 


Tensile 
strength  1  of 

single-ply 
veneer,  14  (d) 
(pounds  per 
square  inch). 


(a) 

Ash,  black  ................................................  120 

Ash,  commercial  white  .....................................  200 

Basswood  ..................................................  200 

Beech  ....................................................  120 

Birch,  yellow  ..............................................  200 

Cedar,  Spanish  ............................................  115 

Cherry  '  ...................................................  115 

Chestnut  .................................................  40 

Cottonwood  ................................................  120 

Cypress,  bald  .............................................  35 

Douglas  fir  ................................................  174 

Elm,  cork  ................................................  65 

Elm,  white  ..........................................  .  ____  160 

Gum,  black  ...............................................  35 

Gum,  cotton  ...............................................  80 

Gum,  red  ..................................................  182 

Hackberry  ......................................................  80 

Hemlock,  western  ..........................................  119 

Magnolia  2  .................................................  40 

Mahogany,  African  3  .......................................  20 

Mahogany,  Philippine  *  ...................................  25 

Mahogany,  true  ...........................................  35 

Maple,  soft  5  ..............................................  120 

Maple,  sugar  ........................  .  .....................  202 

Oak,  commercial  red  .......................................  115 

Oak,  commercial  white  .....................................  195 

Pine,  white  ................................................      •     40 

Poplar,  yellow  ............................................  165 

Redwood  ..................................................  65 

Spruce,  Sitka  ..............................................  103 

Sycamore  .................................................  163 

Walnut,  black  ............................................  110 


(6) 
9.  1 

10.  2 
9.  2 
8.  6 

8.  5 
13.  3 

9.  1 

11.  7 
8.8 

10.  3 

8.  7 

9.  4 

8.  9 
10.  6 
10.  3 

8.7 
10.2 

9.  7 
9.  9 

12.  7 

10.  7 

11.  4 
8.  9 

8.  0 

9.  3 
9.  5 

10.2 

9.  4 

11.  2 

8.  4 

9.  2 
9.  1 


(c) 

0.49 
.60 
.42 
.67 
.67 
.41 
.56 
.43 
.46 
.47 
.49 
.62 
.52 
.54 
.50 
.54 
.54 
.47 
.59 
.52 
.53 
.48 
.57 
.68 
.59 
.64 
.43 
.50 
.41 
.43 
.56 
.59 


6,180 
6,510 
6,880 

13,000 

13,  200 
5,200 
8,460 
4,430 
7,280 
6,560 
6,230 
8,440 
5,860 
6,960 
6,260 
7,850 
6,920 
6,800 

10,000 
5,370 

10, 670 
6,390 
8,180 

10, 190 
5,480 
6,730 
5,640 
7,390 
5,100 
5,600 
8,030 
8,250 


(4 

9,270 

9,770 
10,  320 
19,500 
19, 800 

7,800 
12,  690 

6,645 
10, 920 

9,840 

9,340 
12, 660 

8,790 
10,  445 

9,390 
11, 775 
10, 380 
10,200 
15,000 

8,060 
16,  010 

9,585 
12, 270 
15,  290 

8,220 
10, 095 

8,460 
11, 080 

7,650 

8,400 
12, 045 
12, 375 


*  Specific  gravity  based  on  oven-dry  weight  and  volume  at  test. 

t  Based  on  total  cross-sectional  area. 

t  Based  on  assumption  that  center  ply  carries  no  load. 

Data  based  on  tests  of  3-ply  panels  with  all  plies  in  any  one  panel  same  thickness  and  species. 

'  Probably  black  cherry.        »  Probably  evergreen  magnolia.         »  Probably  khaya  sp.       «  Probably  tanguile. 


Probably  silver  maple. 


SAMPLE   COMPUTATION. 


To  obtain  the  tensile  strength  of  3-ply  wood  consisting  of  two  ^Vincli  birch  faces  and  a  TVinch  basswood  core. 

Parallel  to  face  grain=2X^Xl9,860=l,986  pounds  per  inch  of  width. 

Perpendicular  to  face  grain=lXAX9, 450=591  pounds  per  inch  of  width. 

This  computation  neglects  the  tensile  strength  of  the  ply  or  plies  perpendicular  to  the  grain,  which  is  comparatively 
small.     The  results  are  therefore  slightly  in  error. 

The  resistance  to  splitting  is  of  considerable  importance  in  panels  when  these  are  to  be 
fastened  with  screws  or  bolts  and  are  subject  to  forces  at  the  fastenings.  The  numerical  value 
of  the  work  required  to  split  a  panel  of  a  given  thickness  has  no  direct  application  in  design. 
It  is  only  in  comparison  with  other  panels  of  other  species  or  construction  that  work  in  splitting 
has  any  significance.  The  work  done  is,  of  course,  a  measure  of  resistance  to  splitting.  It  is 
not  entirely  a  property  of  the  wood,  as  it  depends  very  largely  upon  the  strength  of  the  glue. 
98257— 19— No.  12 5 


66 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


If  - 


.98 


S->.      B  * 

&    •%£• 


o     «y  i 


II 


III 

111 


H 

IP 


a  b,s5 
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II 


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a 
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10  co  t-  co  ' 

OS  rH  <N  OO  O  rH 


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a 

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flg 


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ill 

03.4  S  C. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


67 


The  results  of  strength  tests  on  plywood  of  various  common  veneer  species  are  given  in 
table  10.  Except  for  birch  all  tests  are  on  only  one  shipment  of  the  species,  so  that  the  results 
will  in  all  probability  be  changed  somewhat  by  the  addition  of  future  test  data.  The  mahogany 
results  are  on  thin  plywood  ranging  in  thickness  from  ^  inch  to  -fa  inch,  while  the  sizes  of  the 
plywood  for  all  other  species  ranged  from  -^  inch  to  -f  inch. 

In  most  cases  it  was  found  that  the  column-bending  modulus  of  thin  plywood  was  slightly 
less  than  the  column-bending  modulus  of  the  thick  plywood. 

TABLE  11. — Comparison  of  strength  of  3,  5,  and  7  ply  yellow  birch  plywood,  all  plies  of  same  thick- 
ness in  any  one  panel. 


Column-bending   modulus, 
in  pounds  per  square  inch. 

Tension,  in  pounds  per 
square  inch. 

Average 
splitting 

resistance 

Number 
of  plies. 

Average 
specific 
gravity.* 

Average 
per  cent 
moisture. 

Number 
of  tests. 

Parallel.t 

Perpendicu- 
Tar.f 

Parallel.t 

Perpendicu- 
lar.t 

compared  to 
3-ply  birch, 
for  the 
same  ply- 
wood thick- 

ness, in  per 

cent  of  3-ply. 

3 

0.67 

8.5 

195 

16,  000 

3,200 

13,  200 

7,700 

100 

5 

.67 

6.6 

25 

14,  700 

6,800 

13,  100 

8,600 

129 

7 

.70 

7.1 

25 

14,300 

7,900 

12,  900 

9,300 

191 

*  Specific  gravity,  based  on  oven-dry  weight  and  volume  at  test. 

t  Parallel  and  perpendicular  refer  to  direction  of  grain  of  faces  relative  to  direction  of  application  of  force. 

Table  11  shows  the  decrease  in  the  unit  strength  of  plywood  in  the  direction  of  the  grain 
of  the  faces  when  the  number  of  plies  is  increased,  and  the  increase  in  the  unit  strength  of  ply- 
wood perpendicular  to  the  grain  of  the  faces  when  the  number  of  plies  is  increased. 

TABLE  12. — Comparison  of  strength  of  three-ply  wood  having  a  core  of  high  density  with  similar 
plywood  having  a  core  of  low  density  of  the  same  thickness;  each  ply  -fa  inch  thick. 

Number  of  tests  very  limited.    Results  tabulated  will  probably  be  changed  by  further  tests. 


Species. 

Num- 
ber of 
tests. 

Ply- 
wood 
thick- 
ness 

Per 
cent 
mois- 
ture 
at  test. 

Specific 
gravity, 
based 
on 
oven- 
dry 
weight 
and 
volume 
at  test. 

Column-bending 
modulus  in  pounds 
per  square  inch. 

Tension  in  pounds 
per  square  inch. 

load  in  pounds 
per  square  inch, 
5  by  12  inch 
specimen  test- 
ed as  a  column. 

Face. 

Core. 

Face. 

Parallel.* 

Perpen- 
dicular.* 

Parallel.* 

Perpen- 
dicular.* 

Paral- 
lel.* 

Perpen- 
dicu- 
lar.* 

Birch  

Birch  

Birch  

30 
10 
33 
5 
20 
5 
5 

Inches. 
0.15 
.14 
.15 
.15 
.14 
.15 
.14 

9.4 
8.2 
6.9 
7.0 
9.5 
8.3 
6.5 

0.68 
.61 
.69 
.62 
.55 
.44 
.51 

14,200 
15,200 
16,  100 
17,  700 
9,550 
7,200 
10,  100 

3,170 
1,600 
3,210 
2,600 
2,060 
1,400 

11,  900 
12,  900 
9,910 
12,  000 
8,410 
4,900 
6.200 

7,290 
3,800 
6,540 
3,700 
4,720 
3,000 
4,500 

258 
250 
265 
247 
193 
115 
149 

21 
12 
45 
15 
35 
•    11 
17 

Do  
Sugar  maple 
Do  

Basswood  

...do  

Sugar  maple  — 
Basswood.  
Red  gum  

Sugar  maple 
...do  
Red  gum... 
do  

Red  gum... 
Do 

Basswood  

Do 

Yellow  poplar.. 

...do  

*  Directions  refer  to  direction  of  application  of  the  force  relative  to  the  grain  of  the  faces. 

Table  12  shows  that  the  strength  values  of  plywood  parallel  to  the  grain  of  the  faces  are 
practically  the  same  for  three-ply  wood  having  a  core  of  dense  wood  as  for  plywood  having  a 
core  of  light  wood.  The  strength  values  across  the  grain  of  the  faces  are,  however,  very  much 


68 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


less  for  the  plywood  with  core  of  low  density.  In  other  words,  the  strength  values  of  three-ply 
wood  parallel  to  the  grain  of  the  faces  are  almost  entirely  determined  by  the  strength  values 
of  the  face  material,  and  the  strength  values  across  the  grain  of  the  faces  are  very  largely  deter- 
mined by  the  strength  values  of  the  core  species. 

Table  13  gives  a  number  of  factors  that  are  of  value  in  selecting  the  thickness  and  species 
of  the  plies  for  a  three-ply  panel. 

TABLE  13. —  Thickness  jactors  jor  veneer. 

Giving:  (1)  Veneer  thickness  for  the  same  total  bending  strength  as  birch;  (2)  veneer  thickness  for  the  same  weight 

as  birch. 


Species. 

D. 

Average 
specific 
gravity  of 
species  * 
based  on 
oven-dry 
weight  and 
air-dry 
volume. 

Specific 
gravity  of 
giued  ply- 
wood as 
tested. 

Per  cent 
moisture  of 
plywood  as 
tested. 

S. 

Per  cent 
unit  bend- 
ing strength 
compared 
with  birch.t 

Thickness 
factor  for 
the  same 
total  bend- 
ing strength 
as  birch, 

VIoo 
IT 

K,,. 

Thickness 
factor  for 
the  same 
weight  as 
birch, 
0.63 
D  ' 

Ash  black                     

0.50 

0  49 

9  1 

52 

1  39 

1  26 

Ash,  commercial  white  

.58 

.60 

10  2 

72 

1.  18 

1  09 

Basswood        

.38 

.42 

9  2 

48 

1  44 

•      1  66 

Beech  

.63 

.67 

8.6 

94 

1.03 

1.00 

Birch,  yellow  

.63 

.67 

8.5 

100 

1  00 

1.00 

Cedar,  Spanish  

a.34 

.41 

13  3 

43 

1  52 

1  85 

Cherry  &          

.51 

56 

9  1 

80 

1  12 

1  24 

Chestnut  

.44 

.43 

11.7 

34 

1.72 

1.43 

Cottonwood  

.43 

.46 

8  8 

56 

1.34 

1.47 

Cypress,  bald  

.44 

.47 

10.3 

53 

1.37 

1.43 

Elm  cork           

.66 

62 

9  4 

78 

1  13 

95 

Elm,  white  

.51 

.52 

8  9 

58 

1  31 

1.24 

Fir,  Douglas  

.52 

.49 
54 

8.7 
10  6 

60 
56 

1.29 
1  34 

1.24 
1  21 

Gum'  cotton 

52 

50 

10  3 

48 

1  44 

1  21 

Gum,  red  . 

.49 

54 

8  7 

1  25 

1  29 

TT             T    't 

Hackberry  ,  

54 

54 

10  2 

55 

1  35 

1  17 

Hemlock,  western  .   . 

42 

47 

9  7 

60 

1  29 

1  50 

Magnolia  

.51 

58 

9  9 

67 

1  22 

1.24 

Mahogany,  African  'stic 

a.  46 

52 

12  7 

56 

1  34 

1  37 

Mahogany,  Philippine  d  

o  57 

53 

10  7 

68 

1  21 

1  10 

Mahogany,  true  

a  49 

48 

11  4 

57 

1  32 

1  29 

Maple,  soft  e 

48 

57 

8  9 

74 

1  16 

1  31 

Maple,  sugar  

62 

68 

8  0 

100 

1  00 

1  02 

Oak,  commercial  red  

63 

59 

9  3 

59 

1  30 

1  00 

Oak,  commercial  white  

.69 

.64 

9  5 

69 

1  20 

.91 

Pine,  white  

.39 

43 

10  2 

52 

1  38 

1  61 

Poplar,  yellow  

41 

50 

9  4 

58 

1  31 

1  54 

Redwood  

a  36 

41 

11  2 

49 

1  43 

1  75 

Sycamore  

.50 

56 

9  2 

71 

1  09 

1.26 

Spruce,  Sitka  

38 

43 

8  4 

50 

1  41 

1  66 

Walnut,  black  

57 

59 

9  1 

83 

1  10 

1  10 

*  Taken  from  Bulletin  556  of  the  U.  S.  Department  of  Agriculture. 

'  Average  of  the  column-bending  moduli  parallel  and  perpendicular  to  grain  compared  to  birch. 


a  Based  on  subsequent  tests. 
fr  Probably  black  cherry. 


Coast  type  Douglas  fir. 
d  Probably  tanguile. 


Probably  silver  maple. 


The  thickness  factor  (Ks)  is  used  to  obtain  the  thickness  of  a  ply  of  any  species  having 
the  same  total  bending  strength  as  a  given  ply  of  birch.  It  is  arrived  at  as  follows: 

The  strength  of  any  structural  member  is  determined  either  by  the  direct  load  it  can 
sustain  or  the  bending  moment  it  can  resist  without  failure.  In  plywood  the  latter  factor  is 
the  better  criterion  of  strength.  If  we  denote  the  maximum  bending  moment  of  a  strip  of 


Note  12.  AIRCRAFT  DESIGN  DATA. 


three-ply  wood  1  inch  wide  and  of  thickness  dv  by  Mt  and  the  stress  at  failure  by  St  (column- 

Ci     j  n 

bending  modulus),  then  Mt  =  --—-• 

Similarly,  the  strength  of  another  strip  of  a  different  species  will  be  denoted  by  M2,  its 
stress  at  failure  S2,  and  thickness  d2.  By  a  proper  selection  of  thickness  d2  the  second  strip 
may  be  made  to  withstand  the  same  maximum  bending  moment,  so  that  M2  =  Mx  or  S2d22  =  S^2. 

VS 
o*  •    Taking  dt  as  the  unit  of  thickness  of  a  birch  ply- 
Oj 

wood  strip  and  expressing  the  maximum  stresses  in  percentage  of  birch,  we  have  d2  =  -v  a    '  or, 

in  general,  Ks  =  -J-^->  where   Ks  is  the   thickness  of  the  plywood,  whose  column-bending 

modulus  corresponds  to  S  and  whose  total  bending  strength,  given  by  the  bending  moment, 
is  the  same  as  that  of  birch  plywood  of  thickness  unity. 

The  same  reasoning  also  applies  to  single  plies,  so  that  Ks  may  be  used  to  get  the  thickness 
of  a  single  ply,  which  will  give  the  same  total  bending  strength  as  a  birch  ply  of  thickness  unity. 
For  example,  for  yellow  poplar  Ks  =  1.46,  and  a  ply  of  this  species,  1.46X^  =  0.091  inch,  is 
equivalent  in  strength  in  bending  to  a  birch  ply  yg-  mcn  thick. 

By  way  of  explanation  it  must  be  understood  that  unit  bending  strength  refers  to  a  maxi- 
mum stress  such  as  the  modulus  of  rupture,  or  the  column-bending  modulus,  while  total  bending 
strength  refers  to  the  load  or  bending  moment  a  beam  can  sustain  or  the  bending  moment  a 
column  can  sustain. 

It  should  be  kept  in  mind  that  these  factors  will  doubtless  be  modified,  somewhat  by  further 

LGS vS. 

The  thickness  factor  (Kw)  is  used  to  obtain  the  thickness  of  a  ply  of  any  species  equal  in 
weight  to  a  ply  of  yellow  birch  of  given  thickness.  It  is  obtained  by  simply  dividing  the  density 
of  birch  by  the  density  of  the  species  for  which  the  thickness  is  desired.  The  density  data 
used  in  computing  Kw  are  the  same  as  that  given  in  United  States  Department  of  Agriculture 
Bulletin  556,  "Mechanical  Properties  of  Woods  Grown  in  the  United  States."  The  weight  of 
the  glue  in  the  plywood  is  neglected. 

For  yellow  poplar,  for  example,  the  thickness  of  a  ply  equal  in  weight  to  a  y^-inch  ply  of 
birch  is  1.54  X  y^  0.096  inches. 

The  column-bending  tests,  upon  which  the  data  in  table  10  are  based,  were  all  made  on 
specimens  of  the  same  lengths,  and  it  was  felt  desirable  to  determine  what  effect,  if  any,  the 
change  in  length  of  the  column  might  have  upon  the  maximum  unit  load,  the  slenderness  ratio 
remaining  constant.  Special  panels  of  three-ply  birch,  all  plies  of  the  same  thickness  in  each 
panel,  were  made  up  from  veneer  of  the  following  thicknesses:  ^,  ^,  ^r,  yV,  rV>  s>  and  test 
columns  varying  in  length  from  20  inches  to  6  inches  were  cut  from  them  and  tested.  The 
conclusion  drawn  from  these  tests  is  that  for  a  given  slenderness  ratio  the  length  of  the  column 
has  little,  if  any,  effect  on  the  maximum  unit  load  which  a  three-ply  birch  column  will  sustain. 
It  is  assumed  that  the  same  conclusion  will  apply  to  panels  of  other  species. 

Table  9,  to  which  reference  has  already  been  made,  presents  data  by  which  it  is  possible 
to  calculate  the  strength  in  tension  of  plywood  composed  of  various  kinds  of  veneer.  Column 
(d)  of  this  table  is  identical  with  the  corresponding  column  in  table  10.  Column  (e)  is  to  be 
used  in  calculating  the  strength  in  tension  of  plywood  made  up  of  different  species.  The 
method  of  calculation  is  based  upon  the  fact  that  the  tensile  strength  of  wood  in  a  direction 
perpendicular  to  the  grain  is  very  small  in  comparison  with  that  parallel  to  the  grain  and 


70 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


may,  therefore,  for  purposes  of  approximation,  be  neglected.  To  obtain  the  tensile  strength 
in  any  direction,  simply  add  together  the  tensile  strength,  parallel  to  the  grain,  of  the  indi- 
vidual plies  the  grain  of  which  lies  parallel  to  the  direction  in  which  the  strength  is  desired. 
The  sample  computation  will  make  this  entirely  clear. 

The  shearing  strength  of  plywood  is  of  importance  in  connection  with  the  design  of  box 
beams  having  plywood  cheek  pieces  and  for  similar  construction.  Several  series  of  tests  are 
under  way  to  determine  the  shearing  strength  of  plywood  of  various  thicknesses  when  unsup- 
ported for  various  distances.  While  these  tests  are  not  as  yet  completed,  it  is  evident  that 
it  will  not  be  possible  to  use  a  shearing  strength  in  calculating  these  members  much  greater 
than  that  of  solid  wood  of  the  same  species.  There  is  much  more  residual  strength  in  ply- 
wood after  the  first  failure  than  in  solid  wood,  and  for  this  reason  a  somewhat  higher  working 
stress  would  be  justified.  Until  more  data  are  available  the  shear  allowed  in  plywood  should 
not  be  over  25  per  cent  greater  than  that  allowed  in  solid  wood  of  the  same  species.  This 
assumes  that  in  the  cheeks  of  horizontal  beams  the  face  plies  will  be  vertical,  a  condition 
dictated  by  experience  to  be  best  practice. 

RIVETED   JOINTS    IN   PLYWOOD. 

The  matter  of  joints  in  plywood  is  of  the  greatest  importance  in  connection  with  the 
construction  of  various  types  of  built-up  structures  such  as  fuselages,  boat  hulls,  pontoons, 


')([)  imibi: 

-< 

m 

jd 

s 

-  4 

3" 

... 

\ 

Mrsy/iv 

.  3'  \ 

& 

Off// 

?f 

O   O    O    O   O    O    O   < 

)    C 

)    O 

) 
O    O    O   O    O    Q   O    C 

)  ( 

)    O 

, 

s    \ 

Fig.  30. — (a)  Test  specimen  for  single-rivet  tests,     (b)  Test  specimen  for  multiple-rivet  tests. 

and  beams  and  girders.  Several  series  of  tests  have  been  made  to  determine  the  efficiency 
of  various  types  of  joint  for  different  kinds  of  loading. 

The  first  series  of  tests  was  made  upon  riveted  joints  designed  for  tension  and  compres- 
sion. The  tests  were  all  made  in  tension;  both  solid  and  hollow  rivets  were  used.  Two  types 
of  test  were  run;  most  of  the  tests  were  made  on  specimens  only  wide  enough  to  accommo- 
date one  rivet  (fig.  30a),  and  later  enough  wide  specimens  were  tested  (fig.  30b)  to  verify  the 
assumption  that  the  data  on  the  narrow  specimens  could  be  applied  without  correction  to 
wider  ones. 

In  general,  most  of  the  tests  were  made  on  butt  joints,  with  straps  on  each  side.  In  some 
cases  the  straps  were  of  plywood  and  in  others  of  galvanized  sheet  metal  about  0.02  inches 
thick.  The  nomenclature  used  will  become  clear  upon  examination  of  figure  30. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


71 


The  first  tests  were  made  upon  red  gum  plywood  composed  of  three  plies  of  -^  material, 
riveted  with  solid  copper  rivets  through  sheet-metal  cover  plates.  The  grain  of  the  face  plies 
was  perpendicular  to  the  seam.  Figure  31  shows  the  strength  of  the  joint  with  varying  mar- 
gins and  spacing.  It  is  apparent  that  the  best  conditions  are  obtained  with  a  1-inch  margin 
and  a  one-half  inch  spacing. 


6OO 


"';>>.' 


/  /  .  a  +  .  /  //• 

"3f.  ~2  *  '  '•&•  '*£ 


Fig.  31.  —  Single-riveted  butt  joints  in  plywood.  Relations  among  strength,  margin,  and  spacing:  Red  gum  ply- 
wood, plies  1/16  by  1/16  by  1/16  inch;  solid  copper  rivets,  0.15  inch  diameter;  sheet-metal  cover  plates;  grain 
of  faces  perpendicular  to  seam;  moisture,  7.4  per  cent. 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Figure  32  shows  the  variation  of  strength  when  using  a  constant  spacing  of  one-half  inch 
and  margins  varying  from  one-quarter  inch  to  2  inches.  This  figure  shows  very  clearly  that 
no  appreciable  additional  strength  can  be  obtained  by  increasing  the  margin  above  1  inch. 


600 


O.2S        O.SO 


/.OO 

/A/  /MCHES 


/.7S 


2.0O 


Fig.  32. — Single-riveted  butt  joints  in  plywood.  Relation  between  strength  and  margin:  Spacing  1/2  inch;  red 
gum  plywood,  plies  1/16  by  1/16  by  1/16  inch;  solid  copper  rivets,  0.15  inch  diameter;  sheet-metal  cover 
plates;  grain  on  faces  perpendicular  to  seam;  moisture,  7.4  per  cent. 

In  fact,  it  was  found  that  in  case  the  grain  of  the  face  plies  was  parallel  to  the  seam,  the  margin 
could  be  reduced  to  three-quarters  inch  without  sacrificing  an  appreciable  amount  of  strength. 
Similar  tests  made  on  three-ply  birch,  each  ply  one-sixteenth  inch,  gave  similar  results, 
as  shown  in  figures  33  and  34.  With  a  margin  of  1^  inches,  the  maximum  strength  was 
secured  with  a  spacing  of  one-half  inch. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


73 


TOO 


6OO 


k 


300 


I 


/00 


# 


O.2S  O. 

SP/JC//VG 


>v 


% 


O.7S 


/.oo 


Fig.  33. — Single-riveted  butt  joints  in  plywood.  Relation  between  strength  and  spacing:  Margin,  1  1/2  inches; 
birch  plywood,  plies  1/16  by  1/16  by  1/16  inch;  solid  copper  rivets,  0.15  inch  diameter;  sheet-metal  cover 
plates;  moisture,  6.6  per  cent. 


74 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Fig.  3 


O.Z5         a  SO        O.7S 


/.OO         /.2S         /.SO        /.7S 

X/V  //VC//£S 


4. — Single-riveted  butt  joints  in  plywood.  Relation  between  strength  and  margin;  spacing,  1/2  inch;  birch 
plywood,  plies  1/16  by  1/16  by  1/16  inch;  solid  copper  rivets,  0.15  inch  diameter;  sheet- metal  cover  plates; 
moisture,  6.6  per  cent. 

f   ,ni$iBM  :snfcwjqa   im*  msirtroa  n»»wie»d 
iBtem-Jooria    ,-T»Hmi;il>    rfani  3I.O',8J9vh   wqqo-)    biloa  ;ri)ni  ;>t  !  iaiid 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


75 


The  margin  could  have  been  reduced  to  1  inch  or  even  less  without  a  great  falling  off  in 
efficiency.  Figure  35  indicates  that  a  spacing  of  one-half  inch  is  the  best  with  thinner  birch 
(each  ply  -fa  inch). 


ST/?£N6Tff  /N  POUAfaS  P£ft  /MCH  OF  JO/A/r 

1  8  8  t  8  8  1 

/ 

*~~  > 

^ 
S 

b 

/ 

1 

t« 

s 

1 

1 

1 

r 
) 

[4 

V  d 

'  ? 

fOil  ' 

x^^ 

t 

^< 

^ 

^ 

s 

/ 

/ 

< 

^*s 
) 

^  \ 

^^wl 

I 

/ 

/ 

s 

^\ 

^ 

,' 

/ 

^. 

•^ 

/ 

p 

// 

J' 

I 

t 

( 

///  /MCH£S 

Fig.  35.  —  Multiple-riveted  butt  joints  in  plywood;  relation  between  strength  and  spacing;  test  Joint,  5  to  5  1/2  Inches 
wide;  margin,  1  inch;  birch  plywood,  plies  1/20  by  1/20  by  1/20  Inch;  solid  copper  rivets,  0.15  inch  diameter; 
sheet-metal  cover  plates;  moisture,  5.6  per  cent. 


ni  noiiai/boi 


76 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Figures  36  and  37  show  the  strength  of  joints  made  in  three-ply  birch  (each  ply  one-twen- 
tieth of  an  inch)  with  five-eighths-inch  hollow  aluminum  rivets  and  plywood  cover  plates. 
A  spacing  of  \\  inches  gave  the  best  efficiency  with  a  margin  of  2  inches.  It  is  possible  that 


700 


O.S-0 


/.OO 


2.00 


3.00 


/.so 
//v 

Fig.  36. — Single -rive  ted  butt  joints  in  plywood;  relation  between  strength  and  margin;  spacing,  1.25  inches;  birch 
plywood,  plies  1/20  by  1/20  by  1/20  inch;  hollow  aluminum  rivets,  5/8  inch  outside  diameter;  plywood  cover 
plates;  moisture,  5.6  per  cent. 

greater  strength  could  have  been  secured  in  the  case  of  the  specimens  with  the  grain  of  the 
faces  perpendicular  to  the  seam  had  a  greater  margin  than  2  inches  been  used.  In  the  case 
of  the  specimens  with  the  grain  of  the  faces  parallel  to  the  seam  a  margin  of  \\  inches  could 
have  been  used  without  any  great  reduction  in  strength. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


77 


7OO 


/.SO 


2.00 


O.SO  S-OO 

»   boow{iti  naJamflib  ^blnJuo  rfani  £\l 
Fig.  37.  —  Single-riveted  butt  joints  in  plywood;  relation   between  strength   and   spacing;  margin,  2  inches;  birch 
plywood,  plies  1/20  by  1/20  by  1/20  inch;  hollow  aluminum  rivets,  5/8  inch  outside  diameter;  plywood  cover 
plates;  moisture,  5.6  per  cent. 


78 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


The  results  of  tests  upon  three-ply  birch  (each  ply  one-twentieth  inch)  with  plywood 
cover  plates  and  one-half  inch  and  three-eighths  inch  hollow  aluminum  rivets,  respectively, 
are  plotted  in  figures  38  and  39.  These  tests  were  made  with  margins  of  2  inches.  However, 
smaller  margins  could  no  doubt  have  been  used  without  appreciable  loss  in  strength. 


600 


/.SO 


^-j^     C^  V  . 

Fig.  38.— Single-riveted  butt  joints  in  plywood;  relation  between  strength  and  spacing;  margin,  2  inches;  birch 
plywood,  plies  1/20  by  1/20  by  1/20  inch;  hollow  aluminum  rivets,  1/2  inch  outside  diameter;  plywood  cover 
plates;  moisture,  5.6  per  cent. 

•  _ 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


79 


700 


SPA&W&7N  W£tf£5 


/.3& 


Fig.  39.  —  Single-riveted  butt  joints  in  plywood;  relation  between  strength  and  spacing;  margin,  2  inches;  birch 
plywood,  plies  1/20  by  1/20  by  1/20  inch;  hollow  aluminum  rivets,  3/8  inch  outside  diameter;  plywood  cover 
plates;  moisture,  5.6  per  cent. 


80 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


When  the  most  efficient  spacing  and  margin  are  used,  there  is  practically  no  difference  in 
strength  for  the  different  sizes  of  rivets  investigated.  However,  the  smaller  rivets  require  a 
smaller  spacing  and  therefore  more  labor  in  manufacture.  On  the  other  hand,  the  margin 
required  is  less  than  in  the  case  of  the  larger  rivets,  and  this  may  in  some  cases  be  a  decided 
advantage. 

Cover  plates  may  be  of  metal  or  plywood,  as  preferred.  If  of  metal,  aluminum  sheet 
about  three-sixty-fourths  inch  or  one-sixteenth  inch  thick  is  recommended  for  the  thicknesses 
of  plywood  investigated. 

The  efficiency  of  the  joints  was  determined  by  testing  a  number  of  samples  of  the  ply- 
wood, both  parallel  and  perpendicular  to  the  face  plies,  and  it  was  determined  that  under  the 
best  conditions  the  efficiency  of  the  joints  with  the  face  plies  perpendicular  to  the  seam  was 
about  30  per  cent,  while  with  the  face  plies  parallel  to  the  seam  the  maximum  efficiency  was  a 
little  over  50  per  cent. 

While  riveted  joints  may  be  satisfactory  under  certain  circumstances,  they  can  not  be  used 
where  an  efficiency  much  over  50  per  cent  is  required.  In  these  cases  it  is  necessary  to  use 
glued  joints,  of  which  there  are  several  different  types. 


I 

s 

! 

I 

S/sn/3/e  Scarfs/a//?/-  £ticyona/ Scarf Jo/rtf  £//nf>Je  £uff~/o//7/-    D/ayona/ fiutf-Jbtfit   Satv-Joerfih  SuffJoM 

Fig.  40. — Joints  in  the  face  veneer  of  three-ply  wood. 
JOINTS   IN   INDIVIDUAL   PLIES. 

Joints  in  individual  plies  may  be  made  in  a  variety  of  ways.  Figure  40  shows  several 
possible  methods  for  joining  pieces  of  veneer.  A  considerable  number  of  strength  tests  upon 
several  of  these  joints  have  been  made.  The  simple  scarf  joint  has  been  tested  for  a  long  range 
of  slopes  of  scarf.  The  diagonal  scarf  joint,  as  well  as  the  diogonal  butt  joint,  have  been  tested 
for  various  slopes  of  the  diagonal.  The  saw-tooth  butt  joint  has  been  tested  for  various  angles 
of  the  saw  tooth. 

In  balancing  up  the  various  factors  of  strength,  ease  of  manufacture,  and  efficiency  it  was 
decided  that  the  simple  scarf  joint  is  the  most  desirable  of  the  group.  The  simple  butt  joint 
should  not  be  used  where  strength  is  important.  The  edge  joint  is  satisfactory  if  carefully 
made.  The  slope  of  the  scarf  in  the  simple  scarf  joint  should  be  within  the  range  of  from  1  to 
20  to  1  to  30. 

In  comparison  with  the  use  of  rivets,  joints  in  individual  plies  are  probably  more  practical. 
They  have  an  advantage,  too,  in  that  the  joints  in  the  plies  of  a  given  panel  may  be  staggered, 
so  that  any  defect  that  may  occur  in  any  particular  joint  only  partially  weakens  the  entire 
panel.  The  time  and  labor  involved  in  the  preparation  of  this  type  of  joint,  while  probably 
less  than  the  time  and  labor  involved  in  the  preparation  of  riveted  joints,  is  greater  than  that 
in  preparing  the  scarf  joint  extending  through  the  entire  thickness  of  the  panel. 


) 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


JOINTS   EXTENDING   THROUGH   THE   ENTIRE   THICKNESS   OF  PLYWOOD. 

Many  tests  hare  been  made  upon  scarf  joints  extending  through  the  entire  thickness  of  a 
panel.  Such  joints  were  prepared  by  various  manufacturers  using  different  glues,  different 
combinations  of  veneer  thicknesses  and  species,  and  various  slopes  of  scarf.  Two  types  of 
scarf  joints  extending  through  the  entire  plywood  thickness  have  been  tested  and  are  here 
described  as  the  straight  scarf  joint  and  the  Albatros  scarf  joint.  The  two  types  are  shown  in 
figure  41.  The  tests  indicate  quite  conclusively  that  the  straight  scarf  joint  is  the  superior 
joint  of  the  two.  An  examination  of  the  Albatros  joint  will  show  that  the  face  ply  of  the  one 
panel  does  not  meet  the  face  ply  of  the  second  panel  or  only  partially  meets  it.  In  place  of 
being  glued  to  wood  that  has  the  grain  running  in  the  same  direction,  the  face  ply  of  one  panel 
is  glued  to  the  core  of  the  second  panel,  in  which  the  grain  runs  at  right  angles  to  the  grain  in 
the  face.  Joints  in  which  the  grain  of  the  two  pieces  joined  is  at  right  angles  are  not  as  strong 
as  joints  in  which  the  grain  of  the  two  pieces  is  parallel. 


/  Sr?  2O -fa  / //?  3O 


/7//      /  r>  x< 

/J/bafros  ocarf 

Fig.  41. — Joints  in  plywood  extending  through  the  entire  thickness. 

Tension  tests  on  the  straight  scarf  joint  show  that  an  efficiency  of  over  90  per  cent  may 
be  obtained  with  this  type  of  joint  for  a  slope  of  scarf  as  low  as  1  in  10.  On  account  of  the 
variations  hi  the  effectiveness  of  the  gluing  by  different  manufacturers,  it  is  recommended  that 
a  slope  of  scarf  greater  than  this  be  used.  A  slope  in  the  neighborhood  of  1  in  25,  with  a  range 
of  from  1  in  20  to  1  in  30,  is  recommended. 

Severe  weakening  of  scarf  joints  is  often  due  to  sanding  of  the  face  plies  at  the  joint.  Obser- 
vations on  joints  of  this  kind  that  were  sanded  showed  that  at  times  more  than  hah*  of  the  face 
ply  is  ground  away.  Inasmuch  as  the  strength  of  a  panel  lies  almost  entirely  in  the  face  plies 
(in  case  of  three-ply  panels  parallel  to  the  direction  of  the  grain  of  the  faces),  it  is  obvious  that  a 
reduction  in  the  thickness  of  the  face  plies  will  materially  affect  the  strength  of  a  panel.  Con- 
sequently it  is  recommended  that  if  the  scarf  joint  is  sanded  at  all  that  it  be  only  lightly  sanded 
by  hand,  so  as  not  to  decrease  the  thickness  of  the  face  veneer. 

Figure  42  shows  the  method  used  for  cutting  the  scarf  and  for  gluing  the  two  pieces  of 
plywood  together.  The  board  above  the  panel  should  be  relatively  massive  and  flat  so  as  to 
distribute  the  pressure  from  the  screws.  Two  or  three  layers  of  blotting  paper  furnish  sufficient 
padding  to  accommodate  irregularities  in  the  surface. 

98257— 19— No.  12 6  -G. 

gniiaoJ  ni  flonil  rlriw  vMs-iova!  beiaqnioo  ^eifT     .gaomiguoJ  m  wol  fn* 


82 


AIRCRAFT  DESIGN  DATA. 


Note  12 


THIN    PLYWOOD. 

In  an  effort  to  develop  a  substitute  for  linen  for  wing  covering  which  could  be  used  on 
present  types  of  wing  framework,  several  different  kinds  of  thin  plywood  have  been  developed. 
Among  these  are  plywoods  composed  of  three  plies  of  wood,  each  ply  as  thin  as  one  one-hundred- 
and-tenth  inch,  plywoods  with  veneer  faces  and  fabric  cores,  plywoods  with  veneer  faces  and 
metal  wire  core,  plywoods  with  veneer  core  and  cloth  faces,  and  several  other  types.  A  method 
was  developed  which  made  it  commercially  possible  to  glue  up  very  thin  plywood  without 
undue  loss,  although  the  losses  in  making  thin  plywood  are  naturally  much  greater  than  in  mak- 
ing comparatively  thick  plywood  on  account  of  the  fragile  nature  of  the  thin  sheets  and  their 
tendency  to  warp  and  twist  when  glue  is  applied  to  them.  It  was  not  found  possible  to  produce 
a  plywood  having  all  the  requisite  properties  which  was  as  light  as  doped  linen.  The  genera} 
conclusions  drawn  from  the  investigation  follow: 

1.  Spanish  cedar,  mahogany,  birch,  sugar  maple,  red  gum,  yellow  poplar,  black  walnut, 
and  basswood  may  be  cut  into  veneer  sufficiently  thin  for  consideration  in  plywood  air-plane 
wing  covering  as  substitutes  for  linen. 


METHOD  OF  CL/rr/MG  SC/lffF  METHOD  OfP/?£5S/MG  GLUED  JO/NT 

Fig.  42. — Method  of  making  plywood  joints  extending  through  entire  thickness. 

2.  These  species  may  be  glued  satisfactorily  by  the  method  of  introducing  the  glue  between 
the  plies  by  means  of  tissue  paper  previously  coated  with  glue. 

3.  It  does  not  seem  that  plywood  sheets  of  the  same  weight  per  square  foot  as  doped  linen 
can  be  prepared  on  a  practical  scale. 

4.  Covering  made  either  of  veneer  or  of  a  combination  of  veneer  with  fabric,  such  as  linen, 
cotton,  wire  screening,  or  kraft  paper,  in  order  to  be  both  practical  from  the  point  of  view  of 
manufacture  and  satisfactory  in  mechanical  properties  as  shown  by  test,  weighs  from  two  to 
three  times  as  much  as  doped  linen. 

5.  Plywood  that  might  be  considered  practical  from  the  point  of  view  of  manufacture 
possesses  from  two  to  three  times  the  tensile  strength  of  doped  linen. 

6.  The  thinnest  ply-wood  that  can  be  manufactured  at  present  with  any  degree  of  facility 
(3  plies  of  one  one-hundred-and-tenth  inch  Spanish  cedar)  lacks  toughness  and  tearing  strength. 

7.  In  general  the  tearing  strength  of  a  practical  thin  plywood  covering  is  considerably 
higher  than  that  of  doped  linen,  while  its  resistance  to  blows  as  indicated  by  the  toughness  test 
is  lower. 

8.  In  order  to  obtain  the  requisite  degree  of  toughness,  it  is  necessary  to  introduce  a  cloth 
fabric  into  the  construction.     Grade  A  cotton  now  in  use  in  airplane  construction  is  satisfactory 
for  this  purpose. 

9.  Combinations  of  veneer  with  kraft  paper  developed  satisfactory  tensile  strength,  but 
are  low  in  toughness.     They  compared  favorably  with  linen  in  tearing  resistance. 


Note  12.  AIRCRAFT  DESIGN  DATA. 


10.  Combinations  of  veneer  with  light  wire  screening,  thus  far  tested,  are  heavy  and  unsatis- 
factory from  the  point  of  view  of  tensile  strength  per  unit  weight.     Their  toughness  and  tearing 
resistance  are  not  superior  to  cloth  when  used  in  combination  with  veneer. 

11.  Thin  plywood  or  a  combination  of  veneer  with  cloth  is  more  rigid  than  linen. 

12.  Thin  plywood  unprotected  by  a  finish  changes  moisture  content  rapidly  and  shrinks 
or  expands  with  a  change  in  atmospheric  humidity  to  the  extent  of  either  showing  an  appre- 
ciable loosening  or  assuming  a  drum-head  tightness  when  fastened  along  the  edges.     A  finish 
of  three  coats  of  spar  varnish  very  largely  eliminates  rapid  change  in  moisture  content. 

WOVEN   PLYWOOD. 

f 

Tests  have  been  conducted  upon  plywood  made  up  with  basket-weave  faces  and  corru- 
gated core.  The  faces  are  woven  out  of  splints  of  spruce  veneer  1-^  inches  wide  and  0.017 
inch  thick,  while  the  core  is  made  of  spruce  If  inches  wide  and  0.018  inch  thick.  The  total 
thickness  over  all  is  almost  0.2  inch. 

The  following  conclusion  is  drawn  from  the  tests :  The  high  rigidity  at  low  loads,  the  high 
tearing  strength,  stability  under  varying  humidities,  and  comparatively  high  toughness  indicate 
that  the  woven  plywood  tested  may  be  a  very  desirable  material  for  construction  in  airplanes. 

Data  concerning  glues  for  ply-wood  will  be  found  in  the  text  under  the  general  heading 
"Glues." 

The  following  specification  for  waterproof  plywood  is  based  upon  the  strength  tests  just 
described  and  upon  the  glue  tests  presented  farther  on. 

SPECIFICATION   FOR   WATER-RESISTANT   VENEER   PANELS  *OR   PLYWOOD. 

r  c 


GENERAL. 

' 


1.  General  specifications  for  inspection  of  material,  issued  by  the  Bureau  of  Construction 
and  Repair,  in  effect  at  date  of  opening  of  bids,  shall  form  part  of  these  specifications. 

2.  This  specification  covers  the  requirements  for  veneer  panels  for  use  in  aircraft  where 
a  water-resistant  ply-wood  is  specified. 

MATERIALS. 

3.  The  following  species  of  wood  may  be  used  in  plywood  construction: 

^mlKioqab  ^a$«>  ISKJ 
Basswood.  Mahogany  (true  and  African).    Walnut. 

Beech.  Maple  (hard  and  soft.)  Western  hemlock. 

Birch.  Redwood.  White  elm. 

Cherry.  Spanish  cedar.  White  pine. 

Fir  (grand,  noble,  or  silver).  Spruce.  Yellow  poplar. 

4.  Other  species  of  wood  shall  not  be  used  without  the  written  approval  of  the  Bureau  of 
Construction  and  Repair. 

5.  Veneer. — The  veneer  must  be  sound,  clear,  smooth,  well-manufactured  stock,  of  uniform 
thickness  and  free  from  injurious  defects.     Sap  streaks  and  sound  pin  knots  will  not  be  con- 
sidered defects.     Discoloration  will  be  allowed. 

6.  The  veneer  may  be  rotary  cut,  sliced,  or  sawed. 

7.  Thickness. — Unless  otherwise  specified,  no  single  ply  of  veneer  shall  be  thicker  than 
•^  inch.     In  three-ply  stock  the  thickness  of  the  core  must  be  between  40  and  75  per  cent 
of  the  total  thickness  of  the  plywood,  except  for  panels  one-sixteenth  inch  or  less  in  thickness. 

8.  Glue  and  cement. — Any  glue  or  cement  may  be  used  which  will  meet  the  tests  specified 
in  paragraphs  20  and  21. 


84  AIRCRAFT  DESIGN  DATA.  Note  12. 


MANUFACTURE. 


9.  Grain. — The  grain  in  each  ply  shall  run  at  right  angles  to  the  grain  in  the  adjacent 
plies  unless  otherwise  stated  in  the  order. 

10.  Manufacture. — The  plywood  must  have  a  core  of  soft  or  low-density  wood  and  faces 
of  hard  or  high-density  wood  unless  otherwise  specifically  stated  in  the  order.     The  core  may 
be  made  of  several  plies,  in  which  case  the  grain  of  the  adjacent  plies  must  be  perpendicular. 
The  plies  must  be  securely  glued  together,  after  which  the  plywood  must  remain  flat  and  free 
from  blisters,  wrinkles,  lapping,  checks,  and  other  defects.     Plywood  manufactured  with  cold 
glue  must  remain  in  the  press  or  retaining  clamps  not  less  than  three  hours. 

11.  Joints. — Plywood  10  inches  wide  or  less  shall  have  faces  made  of  one-piece  stock.     In 
order  to  conserve  the  narrow  widths  of  veneer,  accurately  made  edge  joints  will  be  allowed  in 
the  faces  and  cores  of  wider  stock,  but  the  number  of  joints  permitted  in  any  ply  shall  not 
exceed  the  width  of  the  panel,  in  inches,  divided  by  eight.     Edge  joints  are  joints  running 
parallel  to  the  grain  of  the  plies  joined.     All  plywood  built  of  jointed  stock  must  be  so  con- 
structed that  all  joints  are  staggered  at  least  1  inch. 

12.  In  panels  over  8  feet  long  scarf  joints  will  be  permitted;  the  smaller  angle  of  the  scarf 
shall  have  a  slope  of  less  than  1  in  25.     Scarf  joints  in  adjacent  plies  must  be  staggered.     Scarf 
joints  are  joints  in  which  the  seam  runs  across  the  ply  at  right  angles  to  the  grain. 

13.  Butt  joints  will  not  be  permitted. 

14.  In  case  the  core  or  crossbanding  is  taped  at  joints  only  unsized  perforated  cloth  tape 
or  open-mesh  unsized  cloth  tape  applied  with  waterproof  glue  or  cement  shall  be  used. 

15.  Moisture  content. -»-The  finished   plywood  shall  be  dried  to  a  moisture  content  of  9 
to  1 1  per  cent,  with  a  tolerance  of  plus  or  minus  2  per  cent,  before  it  is  shipped  from  the  manu- 
facturer's plant.     The  equalization  of  moisture  shall  be  effected  by  kiln  drying,  followed  by 
conditioning. 

16.  Kiln  drying. — The  panels  must  be  piled  and  placed  in  dry  kilns  as  soon  as  possible 
after  being  released  from  the  press.     The  method  of  piling  must  be  approved  by  the  Bureau 
of  Construction  and  Repair.     After  the  stacking  is  completed  the  panels  shall  be  properly 
weighted  to  prevent  warping  during  the  drying  process.     The  best  results  in  the  kiln  are 
obtained  with  a  temperature  of  from  95°  to  115°  F.  and  a  humidity  ranging  from  50  to  60 
per  cent,  depending  upon  the  thickness  of   plywood  and  number  of  plies.     The  circulation 
must  be  maintained  at  all  times. 

17.  Conditioning. — All  panels  must  be  conditioned  before  fabrication  or  shipment.     The 
conditioning  shall  be  done  indoors  under  temperature  and  humidity  conditions  existing  in 
the  factory  for  a  period  of  not  less  than  24  hours  for  three-ply  panels  one-eighth  inch  thick 
and  proportionately  longer  for  thicker  stock.    The  piling  and  weighting  shall  be  the  same  as 
specified  for  dry-kiln  stacks. 

18.  Cutting. — Cutting  for  length  and  width  shall  be  full  and  true.     The  veneer  shall  be 
cut  to  the  thickness  desired  in  the  finished  plywood  and  any  overallowance  on  this  thickness 
for  the  sanding  operation  is  very  undesirable. 

19.  Finish. — In  all  cases  the  tape  must  be  removed  from  the  faces  of  the  panel,  and, 
unless  otherwise  specified  in  the  order,  the  plywood  shall  be  lightly  sanded  to  a  smooth  finish 
free  from  defects. 

TESTS. 

20.  Submission  of  samples  for  test. — The  manufacturer  shall  submit  to  the  Bureau  of 
Construction  and  Repair  for  test  20  samples,  each  1  foot  square,  of  the  plywood  which  he 
proposes  to  furnish  to  airplane  manufacturers. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


85 


21.  Boiling  or  soaking  test. — The  waterproof  quality  of  the  glue  shall  be  tested  either  by 
boiling  in  water  for  a  period  of  eight  hours  or  by  soaking  in  water  at  room  temperature  for 
a  period  of  10  days.     After  boiling  or  soaking  the  samples  shall  be  dried  at  a  temperature 
not  exceeding  150°  F.  to  a  10  per  cent  moisture  content.     The  plies  must  not  separate  when 
the  sample  panels  are  subjected  to  this  test. 

22.  Shear  test. — The  strength  of  the  glue  shall  be  tested  in  five  test  specimens  cut  from 
a  sample  panel.     The  form  of  the  test  specimen  is  shown  in  figure  43.     The  ends  of  the  speci- 
men shall  be  gripped  in  the  jaws  of  a  tension- testing  machine  and  the  load  applied  at  a  speed 
of  less  than  one-half  inch  per  minute.     The  glued  surface  must  not  fail  at  a  load  of  less  than 
150  pounds  per  square  inch. 

23.  Approved  list. — Manufacturers  whose  plywood  does  not   comply  with  these  specifi- 
cations will  not  be  considered  in  awarding  of  contracts.     The  list  of  manufacturers  whose 
product  has  satisfactorily  passed  the  tests  outlined  in  paragraphs  20  and  21  may  be  procured 
from  the  Bureau  of  Construction  and  Repair,  Navy  Department,  Washington,  D.  C. 


- 

JL 

*-         >*    ' 

i          I 
1 
1 
1 

r 
i 
i     i 

li                                                                           -           . 

I 

Plywood    GJue  S/jeor  TesT 
Fig.  43. — Plywood  glue  shear  test  specimen. 

INSPECTION. 


iijl  mo'il 

wiqu  ob^m  alonisq 
ii4  ni  feoul^  ebifi  lo 

;•'  ni  ', 


25.  Unless  otherwise  stated,  all  veneer  and  plywood  shall  be  inspected  at  the  plywood 
manufacturer's  plant. 

26.  The  inspector  shall  make  the  tests  specified  in  paragraphs  21  and  22  on  at  least  one 
sample  panel  from  each  press  for  each  eight-hours'  run. 

27.  In  case  the  plywood  fails  to  meet  the  soaking  and  shear  tests  it  shall  be  rejected.     If 
the  glue  fails  to  meet  one  of  these  tests  but  passes  the  other,  the  test  in  which  it  fails  must 
be  repeated  on  not  less  than  twice  the  original  number  of  specimens  selected  taken  from  two 
or  more  panels.     If  the  glue  fails  to  pass  the  second  test,   the  plywood  represented  by  the 
samples  must  be  rejected. 

28.  In  case  of  consistent  failure  or  lack  of  uniformity  in  product,  the  manufacturer  wiU 
be  required  to  submit  a  detailed  written  statement  giving  the  following  information : 

(a)  The  composition  of  the  glue  and  the  correct  practice  in  mixing  it. 
(&)  The  maximum  time  between  mixing  and  applying  the  glue. 

(c)  The  exact  procedure  in  applying  the  glue  and  in  pressing  and  curing  the  plywoot 
and  such  other  details  as  the  Inspection  Department  may  direct. 
The  inspector  shall  see  that  thereafter  this  schedule  is  observed. 


AIRCRAFT  DESIGN  DATA.  Note  12. 


29.  The  inspector  shall  have  free  access  to  all  parts  of  the  plants  where  the  plywood  is 
being  manufactured  and  shall  be  afforded  every  reasonable  facility  for  inspecting  the  materials 
used,  the  methods  of  manufacture,  and  the  finished  plywood. 

PACKING  AND  SHIPPING. 

30.  Plywood  which  has   passed  inspection  shall  be  packed  in  crates  which  will  protect 
all  edges  and  surfaces  from  injury  during  shipment. 

ORDERING. 

31.  To  facilitate  the  execution  of  contracts  the  order  will  state  any  special  requirements 
which  this  material  must  meet.     The  order  shall  state  the  number  of  pieces,  the  width  across 
the  grain  in  inches,  the  length  with  the  grain  in  inches,  the  thickness  of  the  plywood  and  the 
individual  plies,  the  number  of  plies,  and  the  species  of  wood  to  be  used  for  faces  (to  be 
marked  " Faces"),  for  core  (to  be  marked  "Core"),  and  for  cross-banding  (to  be  marked 
"Crossband").     Sizes  given  shall  be  finished  sizes  and  shall  conform  to  commercial  sizes  when 
practicable.     The  order  shall  also  bear  the  specification  number. 

GLUES  AND  GLUING. 

There  are  a  number  of  distinct  kinds  of  glue  commonly  used  in  aircraft  manufacture.  The 
more  important  of  these  are  as  follows: 

1.  Hide  and  bone  glues. 

2.  Liquid  glues. 

3.  Marine  glues. 

4.  Blood  albumen  glues. 

5.  Casein  glues. 

In  addition  to  these  there  are  many  kinds  of  glue  and  cement  used  in  the  arts  which  are 
not  well  adapted  to  aircraft  uses  and  which,  consequently,  need  not  be  mentioned  here. 

HIDE  AND  BONE  GLUES. 

In  general  only  the  better  grades  of  these  glues  are  used  in  aircraft,  and  these  are  made 
from  hides  and  are  known  simply  as  hide  glues.  Occasionally  nonwater-resistant  plywood 
panels  made  up  with  bone  glue  are  used  in  unimportant  parts  of  aircraft.  The  principal  uses 
of  hide  glues  in  aircraft  have  been  in  laminated  and  spliced  construction  of  various  kinds,  prin- 
cipally in  propeller  manufacture.  Hide  glue  is  still  the  standard  propeller  glue,  though  it  has 
been  replaced  to  an  important  extent  in  other  laminated  work. 

In  order  to  secure  a  very  good  grade  of  glue  for  propeller  and  similar  work,  suitable  methods 
of  testing  were  developed  and  certain  specifications  prepared.  The  Bureau  of  Aircraft  Pro- 
duction regularly  inspects  lots  of  glue  at  the  request  of  manufacturers,  and  glue  passing  the 
required  tests  is  sealed  and  certified.  It  is  then  made  available  for  purchase  by  aircraft  manu- 
facturers, who  are  thus  assured  of  uniform  glue  of  proper  quality.  The  methods  of  test  devel- 
oped and  used  are  given  in  detail  in  the  following  statement.  The  shearing  test  forms  the 
basis  for  the  certification  of  casein  glue  also. 

TESTING   OF   HIDE    GLUE. 

Chemical  analysis  has  been  found  practically  useless  as  a  means  of  testing  glues  because 
of  the  lack  of  knowledge  of  their  chemical  composition.  Physical  tests  must,  therefore,  be 
relied  upon.  A  considerable  number  of  physical  tests  have  been  devised,  some  of  which  are 
important  for  one  class  of  work  and  some  for  another.  For  judging  the  suitability  of  glue  for 
high-grade  joint  work  the  tests  considered  most  important  are  strength,  adhesiveness,  vis- 
cosity, jelly  strength,  odor,  keeping  qualities,  grease,  foam,  and  reaction  to  litmus.  In  the 
subsequent  discussion  of  these  tests  their  application  to  joint  glue  will  be  especially  kept  in  mind. 

|  ,  X    i  .  . 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


87 


Strength  tests  are  made  by  gluing  together  two  or  more  pieces  of  wood  and  noting  the 
pressure  or  pull  required  to  break  them  apart.  Many  different  methods  of  making  the  test 
specimens  and  breaking  them  have  been  devised.  These  depend  to  a  certain  extent  upon  the 
character  of  work  expected  of  the  glue  and  the  nature  of  the  testing  apparatus  available.  The 
simplest  and  most  convenient  strength  test  is  to  glue  two  blocks  together,  as  shown  in  figures 
44  and  45b,  and  shear  them  apart  in  a  timber-testing  machine  (see  fig.  45  a  and  c).  It  will 


MfTHOO  Of PttP/trt/MQ 


Fig.  44. — Method  of  preparing  specimens  for  glue-strength  tests. 

usually  be  found  that  there  is  considerable  difference  in  the  values  obtained  for  the  individual 
specimens.  The  amount  of  difference,  however,  can  be  kept  at  a  minimum  by  using  care  to 
see  that  the  specimens  are  selected,  prepared,  and  tested  under  as  nearly  the  same  conditions 
as  possible.  In  making  strength  tests  the  selection  of  the  wood  is  a  very  important  factor. 
The  species  selected  should  be  the  one  upon  which  it  is  proposed  to  use  the  glue  or  one  fully  as 
strong.  Care  should  be  taken  also  that  the  wood  is  above  the  average  strength  of  the  species, 
in  order  that  there  may  be  less  opportunity  for  the  wood  to  fail  before  the  glue.  If  the  wood  is 
too  weak,  the  full  strength  of  the  glue  is  not  determined. 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


ot  &• 


hji  . 


Note  12.  AIRCRAFT  DESIGN  DATA. 


No  block  should  fail  below  2,200  pounds  per  square  inch,  and  the  average  shearing  strength 
for  a  propeller  glue  should  be  at  least  2,400  pounds  per  square  inch. 

The  viscosity  of  a  glue  is  determined  by  allowing  a  specified  amount  at  a  given  temperature 
to  flow  through  an  orifice.  The  time  required  is  a  measure  of  the  viscosity.  The  time  required 
for  water  to  flow  through  is  taken  as  the  standard.  In  general  it  is  found  that  a  glue  with  high 
viscosity  is  stronger  than  one  with  a  low  viscostiy  and  will  absorb  more  water,  although  there 
are  exceptions.  Hide  glues,  as  a  rule,  have  higher  viscosities  than  bone  glues. 

A  number  of  different  shaped  viscosimeters  have  been  devised.  In  the  glue  manufacturer's 
laboratory,  where  many  tests  must  be  made  each  day,  an  instrument  must  be  used  which  will 
give  results  quickly.  This  can  be  done  with  a  pipette  cut  off  at  one  end  or  with  a  straight 
glass  tube  contracted  at  one  end.  These  instruments  are  not  always  arranged  so  the  tempera- 
ture of  the  glue  within  them  can  be  controlled,  and  for  a  number  of  other  reasons  they  are  not 
entirely  accurate.  Better  control  of  temperature  and  greater  accuracy  can  be  had  with  the 
Engler  viscosimeter.  This  is  more  complicated  and  more  expensive  than  the  glass  tubes  and 
also  slower  to  operate,  but  it  has  the  advantage,  in  addition  to  greater  accuracy,  of  being  an 
instrument  which  is  in  general  use  for  testing  many  kinds  of  materials.  The  values  obtained 
by  its  use  are  readily  understood  by  laboratory  men  and  can  be  readily  checked.  The  instru- 
ment can  be  purchased  standardized  and  ready  for  use. 

The  term  "jelly  strength''  refers  to  the  firmness  or  strength  of  the  jelly  formed  by  a  glue 
solution  of  specified  strength  upon  cooling.  Strong  glues  usually  have  high  jelly  strength. 
There  is  no  standard  instrument  for  determining  jelly  strength  and  no  standard  unit  for  expessing 
it.  In  some  laboratories  the  pressure  required  to  break  the  surface  of  the  jelly  is  measured. 
In  others  the  depth  to  which  a  weight  of  special  shape  will  sink  is  observed.  Sometimes  the 
jelly  is  cast  in  a  conical  shape,  and  the  weight  required  to  press  the  point  of  the  cone  a  certain 
distance  is  taken.  More  common,  however,  is  the  finger  test,  in  which  the  relative  strength 
of  two  or  more  jellies  is  compared  by  pressing  the  jelly  witn  the  fingers.  In  making  this  test 
with  any  apparatus  it  is  important  that  the  conditions  be  very  carefully  controlled  in  order 
that  comparative  results  may  be  obtained.  The  temperature  of  the  jelly  when  tested  is  par- 
ticularly important,  as  the  relative  strength  of  a  number  of  jellies  is  not  the  same  at  different 
temperatures.  In  other  words,  the  jelly  strength  of  the  different  glues  is  not  affected  to  the 
same  extent  by  changes  in  temperature.  The  ideal  condition  is  to  cool  and  test  the  jellies  in 
a  room  constantly  maintained  at  the  proper  temperature.  This  is  seldom  practicable,  how- 
ever, and  the  jellies  must  be  cooled  in  a  refrigerator  and  tested  in  a  warmer  room.  When  this 
is  done  it  is  important  that  the  test  be  made  as  quickly  as  possible  after  removing  the  jelly  from 
the  refrigerator,  so  that  the  temperature  will  be  practically  the  same  as  it  was  in  the  refrigerator. 
The  strength  of  the  glue  solution  must  always  be  the  same  once  a  standard  is  adopted.  For 
high-strength  glues  weaker  solutions  can  be  used  than  for  low-strength  glues. 

The  odor  of  a  glue  is  determined  by  smelling  a  hot  solution  and  gives  some  indication  of 
its  source  or  its  condition.  Glue  which  has  an  offensive  odor  is  not  considered  of  the  highest 
grade.  The  bad  odor  may  be  due  to  the  fact  that  partly  decomposed  stock  was  used  or  that 
the  glue  itself  is  decaying.  For  high-grade  work  it  is  usually  specified  that  the  glue  be  sweet; 
that  is.  it  must  not  have  an  offensive  odor.  The  odor  of  different  glues  varies  considerably, 
and  it  is  difficult  or  impossible  to  express  the  different  "shades."  It  is  usually  not  difficult, 
however,  to  determine  whether  or  not  the  odor  is  clean,  or,  as  it  is  commonly  called,  sweet. 
The  temperature  and  strength  of  solution  are  not  usually  specified. 

The  keeping  quality  of  a  glue  is  determined  by  allowing  the  jelly  left  from  the  jelly-strength 
test  to  stand  in  the  laboratory  at  room  temperature  for  a  number  of  days.  The  odor  and  con- 


90  AIRCRAFT  DESIGN  DATA.  Note  12. 

dition  of  the  glue  are  noted  at  intervals.     Glues  with  good  keeping  qualities  will  stand  several 
days  without  developing  an  offensive  odor  or  showing  any  appearance  of  decomposition. 

For  joint  work  a  small  amount  of  grease  in  glue  is  not  a  serious  objection.  Too  much 
grease,  however,  is  objectionable,  as  grease  has  no  adhesive  properties.  The  grease  can  be 
determined  by  chemical  means,  if  desired,  but  this  is  not  necessary  unless  the  exact  amount 
of  grease  must  be  determined.  The  common  method  of  testing  for  grease  is  to  mix  a  little 
dye  with  the  glue  solution  and  paint  it  upon  a  piece  of  unsized  white  paper.  If  grease  is  present, 
the  painted  streak  will  have  a  mottled  or  spotted  appearance.  If  there  is  no  grease  present, 
the  streak  will  have  a  uniform  appearance. 

Glue  which  foams  badly  is  objectionable  because  air  bubbles  are  apt  to  get  into  the  joint 
and  thus  reduce  the  area  over  which  the  glue  is  in  contact  with  both  faces.  Foamy  glue  is 
especially  undesirable  for  use  in  gluing  machines,  as  in  them  the  glue  is  agitated  much  more 
than  when  it  is  used  by  hand,  and  the  danger  of  incorporating  air  bubbles  is  greater.  The  amount 
of  foam  is  tested  by  beating  the  glue  solution  for  a  specified  time  with  an  egg  beater  or  similar 
instrument  and  then  noting  the  height  to  which  the  foam  rises  and  the  quickness  with  which 
it  siibsides.  Different  laboratories  do  not  make  the  test  in  exactly  the  same  way,  but  in  any 
laboratory  after  a  method  is  once  adopted  it  should  be  strictly  adhered  to  thereafter.  It  is 
common  to  determine  the  foam  on  the  solution  used  in  the  viscosity  test. 

.oul"  Py  its  reaction  to  litmus  a  glue  shows  whether  it  is  acid,  alkaline,  or  neutral.  The  test  is 
made  by  dipping  strips  of  red  and  blue  litmus  paper  in  the  glue  solution  remaining  after  the 
viscosity  test  or  some  other  test  and  noting  the  color  change.  An  acid  glue  turns  blue  litmus 
red,  an  alkaline  glue  turns  red  litmus  blue,  and  a  neutral  glue  will  not  change  the  color  of  either 
red  or  blue  litmus.  A  glue  containing  a  slight  amount  of  acid  is  slightly  preferable  to  one  which 
is  neutral  or  alkaline,  because  it  is  not  quite  so  favorable  a  medium  for  the  growth  of  the  organ- 
isms which  cause  the  decay  of  glue. 

From  the  above  description  of  the  various  glue  tests  it  is  apparent  that,  for  the  most  part, 
they  give  comparative  rather  than  absolute  results.  It  is  rather  difficult  to  compare  the  results 
of  tests  made  by  one  laboratory  with  those  of  another,  as  the  strength  of  solution,  temperature, 
and  manipulation  are  often  different.  For  this  reason  the  most  satisfactory  method  of  pur- 
chasing glues  is  to  specify  that  they  must  be  equal  to  a  standard  sample  which  is  furnished  the 
bidder  to  test  in  any  way  he  sees  fit.  The  bidder  should  also  be  informed  as  to  the  methods  the 
purchaser  proposes  to  use  in  testing  a  glue  submitted  to  him  as  equal  to  the  standard  sample. 
*i((j  iiorfcW  ./HOOT  101  >n  « 

PRECAUTIONS    IN    USING   HIDE    GLUE. 

moll  yJi'H  wU  iMiivoir 

In  using  hide  glue  there  are  a  number  of  precautions  that  must  be  observed  to  obtain  satis- 
factory results.  If  improperly  used,  a  very  high-grade  glue  may  give  poor  joints.  It  is  impor- 
tant, first,  to  find  out  the  right  proportion  of  glue  and  water  to  get  the  best  results.  This  is 
largely  a  matter  of  experience,  but  it  can  also  be  determined  by  strength  tests.  When  the  right 
proportions  are  decided  upon,  they  should  be  strictly  adhered  to  thereafter,  and  the  glue  and 
water  should  be  weighed  out  when  making  up  a  new  batch  of  glue  rather  than  measured  or 
guessed  at.  Clean  cold  water  should  be  put  on  the  glue,  which  should  be  allowed  to  stand  in 
a  cool  place  until  it  is  thoroughly  water  soaked  and  softened.  This  may  take  only  an  hour  or 
it  may  take  all  night,  depending  upon  the  size  of  the  glue  particles.  When  the  glue  is  soft,  it 
should  be  melted  over  a  water  bath  and  the  temperature  not  allowed  to  go  higher  than  about 
150°  F.  High  temperatures  and  long-continued  heating  reduce  the  strength  of  the  glue  solu- 
tion and  are  to  be  avoided.  The  glue  pot  should  be  kept  covered  as  much  as  possible  in  order 
to  prevent  the  formation  of  a  skin  or  scum  over  the  surface  of  the  glue. 


Note  12.  AIRCRAFT  DESIGN  DATA.  91 

The  room  in  which  the  glue  is  used  should  be  as  warm  as  possible  without  causing  too 
much  discomfort  to  the  workmen,  and  it  should  be  free  from  drafts.  In  a  cold,  drafty  room 
the  glue  cools  too  quickly  and  is  apt  to  set  before  the  joint  has  been  put  into  the'  clamps.  This 
results  in  weak  joints.  It  is  also  considered  good  practice  to  warm  the  wood  before  applying 
the  glue.  Wood  should  never  be  glued  when  it  is  cold,  and  of  course  only  thoroughly  seasoned 
wood  should  be  used.  Since  high-strength  animal  glues  set  so  quickly  on  cooling,  they  should 
be  applied  and  the  joints  clamped  as  quickly  as  consistent  with  good  workmanship. 

In  clamping  glued  joints  the  pressure  should  be  evenly  distributed  over  the  joint,  so  that 
the  faces  will  be  in  contact  at  all  points.  The  amount  of  pressure  which  will  give  the  best 
results  is  a  question  which  has  never  been  definitely  settled.  One  experimenter  found  that  a 
pressure  of  about  30  pounds  per  square  inch  gave  better  results  on  end  joints  than  higher  or 
lower  pressures.  Apparently  no  tests  have  yet  been  made  to  show  the  best  pressure  to  use  on 
edge  or  flat  grain  joints.  In  gluing  veneers  it  is  necessary  to  use  high  pressure  in  order  to  flatten 
out  the  irregularities  of  the  laminations.  Pressures  as  high  as  150  or  200  pounds  per  square 
inch  are  sometimes  used. 

Strict  cleanliness  of  glue  pots  and  apparatus  and  of  the  floors  and  tables  of  the  glue  room 
should  be  observed.  Old  glue  soon  becomes  foul  and  affords  a  breeding  place  for  the  bacteria 
which  decompose  glue.  The  fresh  glue  is  therefore  in  constant  danger  of  becoming  contami- 
nated. Glue  pots  should  be  washed  after  every  day's  run  in  hot  weather  and  two  or  three  times 
a  week  in  cooler  weather.  Only  enough  glue  for  a  day's  run  should  be  mixed  at  a  time,  so  that 
mixed  glue  will  not  have  to  be  held  over  from  one  day  to  another.  If  these  sanitary  precau- 
tions are  not  observed,  poor  joints  are  apt  to  be  the  result. 

LIQUID  GLUES. 

Liquid  glues,  frequently  known  as  fish  glues,  have  been  used  to  quite  an  extent  for  the 
smaller  work  such  as  gluing  cap  strips,  tape,  blocks,  moldings,  etc.  They  are  being  replaced 
gradually  by  casein  glues,  which  have  the  advantage  of  water  resistance.  In  general  liquid 
glues  are  not  as  strong  as  certified  hide  glue,  although  the  shearing  strength  of  several  which 
have  been  tested  has  been  as  high  as  2,400  pounds  per  square  inch. 

MARINE  GLUES. 

These  glues  are  used  mainly  to  apply  muslin  between  the  inner  and  outer  skins  of  floats 
and  flying  boat  hulls.  They  are  required  to  be  of  a  sticky,  viscous  nature  and  relatively  non- 
drying  and  elastic.  They  are  usually  composed  of  pine  tar,  rosin,  manila  resin,  and  alcohol. 
On  account  of  their  nondrying  nature,  these  glues  have  comparatively  low  strength.  They 
are  readily  soluble  in  gasoline,  and  it  is  necessary,  therefore,  to  make  provision  to  prevent 
gasoline  from  getting  into  the  bilge  water.  In  general,  marine  glues  are  not  used  to  make  joints 
in  wood  construction  where  high  strength  is  required. 

BLOOD  ALBUMEN  GLUES. 

These  glues,  which  are  made  from  blood  albumen  secured  from  packing  houses,  are  the 
strongest  and  most  water  resistant  of  all  so-called  ''waterproof  glues"  in  common  use  to-day. 
In  general,  it  is  necessary  to  use  heat  (about  225°  F.)  to  set  them,  and  consequently  their  use- 
fulness is  limited  largely  to  plywood  and  similar  thin  material,  although  it  is  possible  to  glue 
thicker  material  in  cases  where  the  proper  heat  can  be  applied  successfully.  Practically  all 
plywood  glued  with  blood  glues  is  glued  between  steam-heated  plates,  which  furnish  a  con- 
venient source  of  heat. 

on  I  hTvrnijfife  ?.•  :-}ib9T§ni 

tttt  \d  bedailqOTCHXNi  od  Y-8"1  Sffixira  eMT    .  •<.[  edi  <xt 


92  AIRCRAFT  DESIGN  DATA.  Note  12. 

Properly  manufactured  blood  albumen  plywood  will  pass  all  the  tests  prescribed  in  the 
plywood  specification  without  difficulty.  Not  only  does  the  shearing  strength  average  far 
above  that  required,  but  the  resistance  to  boiling  and  soaking  is  generally  much  greater  than 
the  specification  requires.  Further,  the  residual  strength  of  the  glue  after  boiling  and  soaking 
is,  in  general,  decidedly  superior  to  that  of  casein  glues. 

A  method  has  recently  been  developed  for  the  gluing  of  very  thin  plywood,  in  which  fine 
tissue  paper  is  impregnated  with  blood  albumen  glue  and  then  dried.  This  tissue  is  then  used 
just  as  ordinary  mending  tissue.  A  sheet  is  placed  between  the  layers  of  veneer  to  be  glued 
and  the  whole  put  under  pressure  between  steam-heated  plates.  Since  the  process  is  a,dry  one, 
the  troubles  due  to  swelling  and  warping  are  eliminated. 

In  general,  it  is  anticipated  that  the  use  of  blood  albumen  glues  will  be  confined  to  manu- 
facturers of  plywood  for  some  time  to  come  and  that  the  only  contact  which  the  aircraft  manu- 
facturer will  have  with  it  will  be  in  the  plywood  which  he  purchases. 

CASEIN  GLUES. 

The  major  ingredient  of  these  glues  is  casein,  a  product  secured  from  the  souring  of  milk. 
Until  a  year  ago  casein  glues  were  hardly  known  in  this  country,  but  they  have  been  developed 
commercially  by  several  concerns,  and  their  use  in  aircraft  has  increased  rapidly.  They  have 
the  advantage  that  they  may  be  used  quite  cold  and  that  no  heat  is  used  either  in  mixing  or 
in  setting  them.  Further,  they  set  up  quickly  but  have  the  disadvantage  of  taking  a  compara- 
tively long  time  to  develop  their  maximum  strength. 

Casein  glue  is  widely  used  in  making  water-resistant  plywood  and  its  use  in  laminated 
construction  (except  propellers)  is  steadily  increasing.  It  is  also  being  used  in  places  where 
formerly  fish  glues  were  mostly  used. 

The  best  grades  of  casein  glue  are  fully  as  strong  as  certified  hide  glue  in  shear,  and  their 
resistance  to  high  humidities  and  to  soaking  is  much  greater.  Tests  now  under  way  indicate 
that  the  shock  resistance  of  casein  glues  is  as  great  as  that  of  certified  hide  glue.  The  technical 
use  of  casein  glue  is  very  simple,  but  it  is  necessary  to  follow  instructions  carefully  in  order  to 
secure  best  results.  The  instructions  which  follow  represent  the  best  practice  and  are  based 
upon  experience  both  in  laboratory  and  in  the  shop. 

INSTRUCTIONS    FOR    USE. 

Equipment. — In  using  waterproof  casein  glues  the  mixers  used  ordinarily  for  animal  glue 
and  vegetable  glue  are  generally  not  very  successful,  as  a  more  rapid  and  thorough  stirring 
than  these  mixers  give  is  usually  necessary.  It  is  possible  that  some  types  of  ordinary  glue 
mixers  can  be  speeded  up  enough  to  give  good  results  with  casein  glues,  but  they  have  additional 
disadvantages  in  being  rather  difficult  to  keep  clean.  The  most  successful  mixer  so  far  found 
for  these  glues  is  the  power  cake  mixer,  such  as  is  used  by  bakers,  or  machines  constructed  on 
a  similar  plan.  These  machines  have  several  speeds  and  mix  the  glue  in  a  detachable  kettle 
which  is  easily  cleaned.  They  can  also  mix  relatively  small  quantities,  so  that  no  batch  of 
glue  needs  to  stand  very  long  before  being  used  up.  Copper,  brass,  or  aluminum  vessels  should 
not  be  used  for  mixing  casein  glues,  as  the  alkali  in  the  glues  attacks  these  metals.  It  is  advisable 
also  to  equip  the  glue  pot  with  a  metal  hood  fitted  with  a  feed  hopper  in  order  to  prevent  spat- 
tering outside  of  the  glue  pot  during  the  course  of  mixing. 

Preparation  of  glue. — It  is  advisable,  in  all  cases,  to  thoroughly  mix  the  contents  of  a 
freshly  opened  barrel  of  prepared  glue,  and  preferably  several  barrels  should  be  mixed  at  once 
before  any  of  the  dry  powder  is  withdrawn  for  use,  in  order  to  counteract  the  segregation  of 
ingredients  of  varying  specific  gravities  which  may  have  occurred  during  shipment  from  the 
factory  to  the  point  of  consumption.  This  mixing  may  be  accomplished  by  transferring  the 


Note  12.  AIRCRAFT  DESIGN  DATA.  93 

contents  of  the  barrels  to  a  box  of  suitable  size  in  which  the  dry  glue  is  turned  over  a  sufficient 
number  of  times  and  thoroughly  mixed  with  a  clean  shovel. 

It  is  necessary  to  caution  against  the  practice  observed  in  some  plants  of  sifting  the  pow- 
dered glue  and  discarding  from  it  the  coarse  matter  which  remains  upon  the  screen.  This 
may  remove  from  the  glue  an  essential  ingredient  and  thus  defeat  the  purpose  for  which  the 
glue  is  intended. 

Proportions  of  dry  glue  and  water. — The  proportion  of  water  to  mix  with  the  dry  glue 
should  be  as  directed  by  the  glue  manufacturer.  It  is  to  be  borne  in  mind,  however,  that 
fixed  proportions,  satisfactory  for  each  and  every  barrel  of  glue  received,  can  not  be  speci- 
fied because  of  a  slight  lack  of  uniformity  which  may  exist  in  the  product.  Hence,  only 
average  proportions  can  be  stipulated  by  the  manufacturer,  and  the  operator,  in  order  to 
obtain  satisfactory  consistencies,  may  find  it  necessary  at  times  to  vary  from  the  average 
proportions  specified.  It  has  been  found  in  some  cases  that  using  exactly  the  same  propor- 
tions of  glue  and  water,  the  glue  from  one  barrel  may  be  thinner  than  that  from  another.  It 
is  hoped  that  this  difficulty  will  be  overcome  before  long  by  improved  manufacturing  methods, 
but  until  it  is  much  will  have  to  depend  upon  the  judgment  of  the  operator.  It  should  also 
be  remembered  that  some  classes  of  work  require  thicker  glue  than  others. 

Mixing  the  glue. — The  correct  quantity  of  water  is  placed  in  the  glue  pot  and  the  mixing 
blade  is  brought  into  action  at  proper  speed.  A  high  speed  is  necessary  at  first,  especially 
if  the  glue  is  not  added  to  the  water  very  slowly,  in  order  to  avoid  the  formation  of  lumps  in 
the  glue.  There  is  a  considerable  range  of  speed,  however,  which  will  give  satisfactory  results. 
In  some  cases  a  speed  of  140  revolutions  per  minute  of  the  shaft  which  carries  the  mixing  blade 
(about  350  revolutions  per  minute  of  the  blade  itself)  is  used  satisfactorily.  By  adding  the 
glue  carefully,  however,  a  speed  as  low  as  80  revolutions  per  minute  of  the  vertical  shaft  (180 
revolutions  per  minute  of  the  blade)  can  be  successfully  used.  The  powdered  glue  is  now 
slowly  introduced  through  the  feed  hopper  and  the  agitation  is  allowed  to  continue  for  about 
five  minutes  and  then  stopped. 

The  sides  of  the  glue  pot  should  now  be  scraped  in  order  to  direct  any  of  the  spattered 
material  into  the  mixture,  whereupon  the  blade  is  again  brought  into  action  at  reduced  speed 
(60  to  90  revolutions  per  minute)  for  a  period  of  at  least  ten  minutes.  The  object  of  reducing 
the  speed  after  the  first  stage  of  mixing  is  to  prevent  the  incorporation  of  an  excess  of  air.  At 
the  end  of  this  stirring  period  the  glue  is  ready  for  use,  provided  all  the  fine  casein  particles 
are  dissolved  and  no  appreciable  amount  of  air  has  been  whipped  in.  If  the  glue  still  con- 
tains fine  particles  of  undissolved  casein  and  has  the  appearance  of  "cream  of  wheat"  mush, 
however,  the  mixture  should  be  continued.  It  was  formerly  considered  necessary  to  allow 
the  glue  to  stand  without  stirring  for  a  short  period  before  using  it.  The  object  of  this  was 
to  allow  all  the  casein  to  dissolve.  It  has  now  been  found,  however,  that  it  is  better  practice 
to  accomplish  this  solution  by  continued  mixing  than  by  standing.  If,  however,  it  is  found 
that  air  bubbles  have  been  whipped  into  the  glue  during  mixing,  it  is  desirable  to  let  it  stand 
awhile  so  the  air  can  separate. 

In  mixing  casein  glues  which  may  require  the  addition  of  different  ingredients  singly 
the  above  practice  should  be  varied  from  to  conform  with  the  directions  of  the  manufacturer. 

Consistency  of  glue. — It  may  be  found  that  the  proportions  used  do  not  always  give  exactly 
the  same  consistency.  So  long  as  the  glue  is  neither  too  thick  nor  too  thin  to  spread  well, 
however,  slight  differences  in  consistency  between  individual  batches  or  shipments  of  glue 
need  not  be  considered  serious.  Good  results  may  be  expected  if  the  glue  spreads  properly. 
Other  things  being  equal,  thick  mixtures  develop  higher  strength  than  thin  mixtures,  and 
when  great  strength  is  desired  it  is  desirable  to  use  the  thickest  mixtures  practicable. 


AIRCRAFT  DESIGN  DATA.  Note  12. 


If  in  mixing  up  a  batch  of  glue  from  a  new  barrel  or  shipment  of  some  kinds  of  glue  it  is 
found  that  the  proper  consistency  is  not  obtained,  it  is  possible  to  alter  it  if  attended  to  imme- 
diately and  before  the  glue  has  been  removed  from  the  mixing  pot.  This  should  not  be 
attempted  on  important  work  unless  the  operator  fully  understands  his  glue,  and  it  should  be 
entirely  avoided  if  possible. 

If  the  glue  mixture  obtained  is  seen,  before  it  is  taken  from  the  mixing  pot,  to  be  too  thick 
to  spread  properly,  it  can  be  thinned  by  adding  an  extra  part  or  two  of  water,  as  may  be  required, 
and  stirring  at  slow  speed  until  the  water  is  thoroughly  incorporated.  This  holds  for  any 
casein  glue.  Under  no  circumstances,  however,  should  water  be  added  to  glue  which  has 
thickened  on  standing  or  after  being  used  awhile. 

If  the  glue  mixture  is  found,  before  removing  from  the  mixing  pot,  to  be  too  thin,  it  may 
be  thickened  by  carefully  adding  a  proper  amount  of  dry  glue  with  continued  stirring.  This 
is  practicable  only  for  glue  in  which  all  the  ingredients  are  mixed  together  dry,  and  is  not  suit- 
able for  glues  in  which  the  various  ingredients  are  added  separately.  The  stirring  should  then 
be  continued  long  enough  to  dissolve  all  the  casein  of  the  added  glue.  Another  method  which 
might  be  used  is  to  mix  a  thicker  batch  of  glue  and  then  mix  the  two  batches  together.  It 
is  far  preferable  to  avoid  using  either  method,  and  with  proper  care  it  should  seldom  be  found 
necessary. 

Application  and  use  of  glue. — The  glue  in  any  batch  should  be  used  up  completely  before 
it  begins  to  thicken  materially.  The  length  of  time  during  which  the  mixed  glue  can  be  success- 
fully used  may  vary  with  different  shipments.  The  operator  must  judge  whether  or  not  the 
glue  is  fit  to  use  at  any  time  by  its  consistency.  Tests  have  shown  that  good  results  may  be 
expected  from  a  normal  glue  at  any  time  during  its  working  life  up  to  the  time  when  it  becomes 
too  thick  to  spread  properly. 

In  spreading  the  glue  it  is  important  that  enough  be  applied  to  coat  all  the  surface  of  both 
faces  of  the  joint.  An  appreciable  amount  of  glue  should  squeeze  out  of  the  joints  when  pres- 
sure is  applied.  As  little  time  as  possible  should  elapse  between  the  spreading  of  the  glue  and 
the  pressing.  The  exact  time  which  can  safely  elapse  will  vary  with  the  kind  of  wood  being 
used,  the  consistency  of  the  glue,  the  amount  of  glue  applied,  the  temperature,  and  other  factors. 
In  making  veneer  panels  it  is  considered  best  practice  to  get  the  stack  under  pressure  within 
ten  minutes  or  less  from  the  time  the  first  ply  is  spread. 

The  minimum  time  the  joints  must  be  left  under  pressure  is  not  known.  It  is  considered 
safest  and  best  practice,  howerer,  to  leave  the  joints  in  the  press  or  in  retaining  clamps  for  at 
least  three  hours.  After  the  glued  material  is  taken  from  the  press  it  should  be  dried  either 
artificially  or  naturally  to  remove  the  moisture  added  by  the  glue.  It  is  best  also  to  allow  the 
material  to  stand  a  week  or  two  to  develop  the  full  strength  and  water  resistance  of  the  glue. 
The  panels  should,  of  course,  be  piled  properly  during  the  drying  period  to  prevent  warping. 

The  above  discussion  is  applicable  in  general  to  casein  glues,  whether  of  the  prepared  type, 
such  as  Certus,  Napco,  Casco,  or  Perkins  waterproof  glue,  or  of  the  type  which  is  mixed  by 
the  user  directly  from  the  raw  materials. 

The  following  points  should  be  kept  in  mind  in  preparing  and  using  casein  glues: 

(1)  Thoroughly  mix  each  barrel  of  glue  before  using. 

(2)  Weigh  the  glue  and  water;  do  not  measure  it. 

(3)  Avoid  lumpy  mixtures. 

(4)  Avoid  mixtures  which  are  too  thick  or  too  thin. 

(5)  Mix  until  all  the  fine  particles  dissolve  and  a  smooth  mixture  is  obtained. 

(6)  Do  not  use  glue  after  it  becomes  too  thick  to  spread  properly. 

(7)  Do  not  attempt  to  thin  or  thicken  glue  after  it  leaves  the  mixer. 


Note  12.  AIRCRAFT  DESIGN  DATA. 


DIRECTIONS   FOR   MIXING   CERTUS    GLUE. 

In  general  use  about  10  parts  of  glue  and  17  to  20  parts  of  water.  Both  water  and  glue 
should  be  weighed,  not  measured.  With  the  water  in  the  mixing  can,  start  the  mixing  blade 
at  high  speed  (80  to  140  revolutions  per  minute  of  the  vertical  shaft  is  about  right)  and  add 
the  dry  glue  rather  slowly.  Continue  this  rapid  stirring  for  about  3  to  5  minutes  after  the  last 
dry  glue  is  added;  then  stop  the  mixer,  scrape  down  the  sides  of  the  can,  and  start  mixing  at 
slow  speed  (40  to  60  revolutions  per  minute  of  the  vertical  shaft  is  about  right).  After  10  to 
15  minutes  at  slow  speed  the  glue  should  be  ready  for  use.  If  it  has  a  granular  appearance 
at  the  end  of  this  time,  however,  the  casein  is  not  all  dissolved,  and  mixing  should  be  continued 
long  enough  to  get  casein  particles  into  solution.  The  glue^  is  then  ready  to  use. 

DIRECTIONS   FOR   MIXING   NAPCO   GLUE. 

In  general  use  about  10  parts  of  glue  and  17  to  20  parts  of  water.  Both  water  and  glue 
should  be  weighed,  not  measured.  With  the  water  in  the  mixing  can,  start  the  mixing  blade 
at  high  speed  (80  to  140  revolutions  per  minute  of  the  vertical  shaft  is  about  right)  and  add  the 
dry  glue  rather  slowly.  Continue  this  rapid  stirring  for  about  3  to  5  minutes  after  the  last 
dry  glue  is  added,  then  stop  the  mixer,  scrape  down  the  sides  of  the  can,  and  start  mixing  at 
slow  speed  (40  to  60  revolutions  per  minute  of  the  vertical  shaft  is  about  right).  After  about 
30  minutes  at  slow  speed  the  glue  should  be  ready  for  use.  If  it  has  a  granular  appearance  at 
the  end  of  this  time,  however,  the  casein  is  not  all  dissolved,  and  mixing  should  be  continued 
long  enough  to  get  the  casein  particles  into  solution.  The  glue  is  then  ready  to  use. 

DIRECTIONS   FOR   MIXING   CASCO    GLUE. 

Before  starting  any  mixing  weigh  out  all  ingredients,  using  the  following  proportions: 

.  ( Water,  22£  parts. 

I  Prepared  Casco  casein,  10  parts. 
RJ  Water,  1  part. 

1  Caustic  soda,  £  part. 
p(  Water,  5  parts. 

IHydrated  lime,  5  parts. 

With  the  water  of  A  in  the  mixer  and  paddle  operating  at  an  intermediate  speed  (in  the 
neighborhood  of  60  to  90  revolutions  per  minute  of  the  vertical  shaft  of  a  cake  and  dough  mixer) 
slowly  add  the  casein  and  continue  stirring  till  the  mass  is  free  from  lumps.  This  should  require 
about  3  or  4  minutes. 

Now  slowly  add  the  one-half  part  of  caustic  soda  which  has  been  previously  completely 
dissolved  in  the  1  part  of  water,  and  continue  stirring  for  about  3  minutes. 

Next  add  the  5  parts  of  hydrated  lime  which  has  previously  been  worked  into  a  smooth 
paste  with  the  5  parts  of  water,  and  continue  stirring  until  a  smooth  mixture  free  from  lumps 
and  undissolved  particles  of  casein  is  obtained.  This  should  require  about  15  minutes,  possibly 
a  little  longer.  The  glue  is  now  ready  for  use.  If  it  is  found  that  any  appreciable  quantity  of 
air  has  been  incorporated  into  the  glue  by  the  stirring,  the  glue  should  be  allowed  to  stand  10 
to  20  minutes  before  using  to  allow  the  air  to  escape. 

Glue  mixed  according  to  the  above  procedure  is  ordinarily  considered  satisfactory  for 
gluing  veneer  one- twelfth  inch  thick  or  thinner.  If  the  glue  appears  too  thin,  however,  it  can 
be  made  thicker  by  using  less  water,  as  suggested  below.  For  joint  work  or  thicker  veneer 
also  a  somewhat  thicker  consistency  is  desirable.  This  can  be  obtained  by  using  17  to  20  parts 
of  water  under  A  instead  of  22$  parts. 


AIRCRAFT  DESIGN  DATA.  Note  12. 


DIRECTIONS    FOR   MIXING   PERKINS    WATERPROOF   CASEIN   GLUE. 
(As  recommended  by  the  manufacturer  September,  1918.) 

When  the  paddle  itself  is  running  about  400  revolutions  per  minute,  the  following  method 
is  highly  satisfactory  for  making  up  "P.  W.  G."  into  finished  glue: 

Dissolve  1  pound  of  76  per  cent  caustic  soda  in  30  pounds  of  water  contained  in  the  large 
bowl.  Add  14  pounds  of  "P.  W.  G."  slowly  to  the  caustic  solution  with  thorough  and  brisk 
agitation.  Continue  agitation  for  about  5  minutes.  Allow  the  glue  to  stand  20  to  30  minutes 
after  mixing  before  using. 

When  the  speed  of  the  paddle  itself  is  less  than  400  revolutions  per  minute  the  following 
method  will  give  a  smooth,  fine  flowing  batch: 

Add  14  pounds  of  "P.  W.  G."*to  27  pounds  of  water.  Agitate  to  smooth  consistency. 
Continue  agitation  and  add  in  small  portions  a  solution  made  by  dissolving  1  pound  of  caustic 
soda  in  3  pounds  of  water.  Continue  agitation  for  about  5  minutes  after  ingredients  are  all  in. 
Allow  to  stand  20  or  30  minutes  after  mixing  before  using. 

Neither  casein  nor  blood  albumen  glues  seem  to  be  affected  by  gasoline  in  the  slightest 
degree.  A  number  of  panels  made  up  by  representative  manufacturers  were  soaked  for  a  long 
period  (several  months)  in  gasoline  without  any  sign  of  deterioration.  Similar  panels  were  also 
soaked  for  a  like  period  in  gas  engine  oil  (Polarine)  without  any  apparent  deterioration.  These 
tests  indicate  that  both  blood  albumen  and  casein  plywoods  can  be  used  around  the  engine 
without  fear  of  damage  by  gasoline  and  oil. 

Frequently  it  becomes  desirable  to  fill,  shellac,  or  varnish  parts  which  are  later  to  be  glued. 
Tests  made  to  determine  the  strength  of  joints  made  on  wood  treated  in  this  manner  show 
that  they  are  very  weak  and  absolutely  unreliable.  No  joints  in  aircraft  work  should  be  made 
except  with  the  bare  wood.  ,rfo  ^ 

AIRCRAFT  PARTS. 

On  account  of  the  impossibility  of  computing,  with  any  degree  of  accuracy,  the  strength 
of  many  aircraft  parts  and  assemblies,  it  has  been  found  necessary  to  supplement  the  designs 
and  calculations  with  actual  test  to  destruction.  The  tests  have  frequently  shown  unexpected 
weak  points,  which  have  been  strengthened  and  the  parts  retested.  Through  development  of 
this  character  some  very  remarkable  results  have  been  achieved,  and  the  way  has  been  opened 
for  similar  work  along  allied  lines. 

LAMINATED  CONSTRUCTION. 

One  of  the  first  problems  to  come  up  in  this  connection  was  a  study  of  laminated  wood 
construction.  Opinion  concerning  the  merits  of  this  type  of  construction  have  been  divided 
for  a  long  time,  and  designers  have  allowed  their  fancy  free  reign  in  devising  widely  varying 
styles  of  built-up  members.  Until  about  a  year  ago  designers  were  allowed  to  use  either  solid 
or  built-up  construction,  in  accordance  with  their  individual  needs  or  desires,  but  during  the 
present  year  there  has  been  a  very  decided  trend  toward  official  insistence  upon  laminated 
construction  in  preference  to  solid,  especially  in  the  case  of  wing  spars.  There  are  several 
reasons  back  of  this  trend,  not  the  least  important  of  which  is  the  increasing  difficulty  of  securing 
large  sizes  in  the  desired  grades.  In  the  case  of  propellers  lamination  has  been  practically  uni- 
versal for  many  years. 

While  lamination  undoubtedly  does  promote  the  use  of  smaller  and  shorter  material,  with 
the  consequent  better  utilization  of  lumber  and  does  insure  the  elimination  of  large  hidden 
defects,  it  requires  the  exercise  of  a  great  deal  of  care  to  insure  satisfactory  results.  The  prin- 
cipal difficulties  encountered  lie  in  the  warping  and  twisting  of  the  finished  part.  The  relations 


Note  12.  AIRCRAFT  DESIGN  DATA.  97 

existing  between  shrinkage  and  moisture,  density,  and,  direction  of  grain  have  already  been 
discussed  in  detail.  Let  it  suffice  to  say  that  unequal  shrinkage,  with  consequent  twisting  or 
warping,  will  result  in  a  laminated  structure  if  the  various  laminations  differ  materially  from 
each  other  in  any  of  the  three  factors  mentioned,  namely,  moisture,  density,  and  direction 
of  grain. 

Propellers  probably  need  as  much  care  in  their  manufacture  as  any  aircraft  parts  in  order  to 
insure  permanence  of  pitch,  balance,  etc.  The  following  rules  for  the  selection  of  wood  for 
laminated  construction  have  been  prepared  especially  for  propeller  manufacture,  though  they 
apply  in  general  to  all  laminated  construction: 

(1)  All  material  should  be  quarter-sawed  if  possible. 

(2)  Quarter  and  flat-sawed  laminae  should  not.be  used  in  the  same  propeller. 

(3)  All  laminae  should  be  brought  to  the  same  moisture  content  before  gluing  up. 

(4)  All  laminae  in  the  same  propeller  should  have  approximately  the  same  specific 

gravity. 

(5)  All  laminae  in  the  same  propeller  should  be  of  the  same  species. 

Dry  wood  when  exposed  to  very  humid  air  absorbs  moisture  and  swells.  Wood  dried  in  a 
normally  dry  atmosphere  till  its  moisture  content  becomes  practically  constant  loses  moisture, 
and  shrinks  when  exposed  to  extremely  dry  conditions.  Two  pieces  of  wood  when  exposed  con- 
tinuously to  the  same  environment  will  eventually  come  to  practically  the  same  moisture  con- 
tent, irrespective  of  their  relative  moisture  contents  when  first  exposed  to  this  environment. 

Individual  pieces  of  wood,  even  those  of  the  same  species,  vary  greatly  in  their  rate  of 
drying.  Quarter-sawed  pieces  have  a  different  drying  rate  from  plain-sawed  pieces.  Dense 
pieces  dry  more  slowly  than  those  which  are  less  dense. 

Suppose  that  a  flat-sawed  board  is  glued  between  two  quarter-sawed  boards,  all  three 
having  the  same  moisture  content,  say,  15  per  cent,  when  glued  up;  or,  suppose,  that  under 
similar  conditions  a  very  dense  piece  is  glued  between  two  pieces  which  are  less  dense;  or,  sup- 
pose that  a  board  containing  15  per  cent  moisture  is  glued  between  two  others,  each  containing 
10  per  cent  but  all  three  being  of  the  same  density  and  cut  in  the  same  manner.  Then  suppose 
the  finished  product  to  be  dried  to,  say,  8  per  cent  moisture.  Every  piece  will  shrink,  but  in 
each  instance  the  center  piece  will  tend  to  shrink  more  than  the  outside  ones.  The  glued  joint 
will  be  under  a  shearing  stress,  since  the  center  piece  has  a  tendency  to  move  with  respect  to 
those  on  the  outside.  Under  this  condition  the  glued  joint  may  give  way  entirely,  it  may 
partially  hold,  or  it  may  hold  perfectly.  In  either  of  the  latter  cases  the  center  piece  will  be 
under  stress  in  tension  across  the  grain,  and  consequently  will  have  a  tendency  to  split.  This 
tendency  may  become  localized  and  result  in  visible  splitting  or  it  may  remain  distributed  and 
cause  a  lessening  of  the  cohesion  between  the  wood  fibers,  but  without  visible  effect. 

If  a  combination  of  these  three  cases  occurs,  it  may  be  much  more  serious  in  its  effect  than 
any  one  alone.  For  instance,  suppose  that  in  a  propeller  alternate  laminations  are  of  flat- 
sawed,  dense  boards,  glued  at  a  relatively  high  moisture  content,  while  the  others  are  quarter- 
sawed,  less  dense,  and  at  a  much  lower  moisture  when  glued.  The  tendency  of  the  flat-sawed 
laminations  to  shrink  will  be  very  much  greater  than  that  of  the  others,  with  the  result  that 
internal  stresses  of  considerable  magnitude  will  be  set  up. 

It  is  not  difficult  to  see  how  these  internal  stresses  may  combine  with  the  stresses  from 
external  causes  and  with  the  continual  vibration  to  produce  failure  under  external  loads  which 
are  considerably  smaller  than  the  propeller  would  safely  resist  if  manufactured  with  proper  care. 

In  the  case  of  laminated  struts  and  beams  the  laminations  should  be  matched  as  to  direc- 
tion of  annual  rings,  as  they  appear  on  the  end  section,  to  balance  shrinkage  as  much  as  possible. 

98257— 19— No.  12 7 

.no:  i  Iliv/  ,!>'»n!;j  Ynsqo'Kf 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Quarter-sawed  and  flat-sawed  material  should  never  be  used  in  the  same  member.  Neither 
should  either  quartered  or  flat  stock  be  used  with  stock  cut  at  intermediate  angles.  In  lami- 
nating together  pieces  cut  with  the  annual  rings  at  an  angle  of  about  45  degrees  with  the 
faces  the  rings  in  the  adjacent  laminations  should  be  approximately  perpendicular  to  each 
other  instead  of  approximately  parallel  to  each  other. 

WING  BEAMS. 

In  order  to  determine  the  general  principles  underlying  the  design  of  built-up  wing  beams, 
to  develop  the  best  forms  from  the  standpoints  of  efficiency,  utilization  of  low-grade  stock,  and 
ease  of  manufacture,  and  to  study  problems  connected  with  manufacture,  a  series  of  300  beams 
of  various  types  and  designs  were  built  and  tested.  These  types  included  only  those  which 
gave  promise  of  strength  efficiency  combined  with  utilization  of  smaller  material  than  that 
needed  for  the  manufacture  of  solid  beams,  since  the  problem  at  the  time  was  primarily  one 
of  shortage  of  material.  The  types  selected  besides  the  solid  ones  used  for  comparison  are 
shown  in  figure  46. 


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To 

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V- 

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J 

LJ 

Veneer  cheeks,  birch 


'/3  andlfo"  /aminafions   faces  and  ' 


Po/o/ar  veneer 
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hms 


Fig.  46. — Cross  sections  of  built-up  test  beams. 


While  it  is  impossible  at  the  present  time  to  present  detailed  analyses  of  these  tests,  the 
general  conclusions  drawn  from  them  are  given  in  the  following  statements: 

The  tests  were  divided  into  various  series  for  ease  in  reference,  each  series  representing 
different  conditions  from  the  others.  The  conclusions  from  each  series  are  first  given,  with 
the  general  conclusions  at  the  end. 

RESULTS    OF    VARIOUS    BEAM    TESTS. 

Series  1  and  12  (fig.  46e  and  f):  These  consisted  of  one-piece  spruce  beams  of  acceptable 
material  compared  with  three-piece  beams  of  similar  matched  material.  The  results  compared 
favorably,  although  with  the  built-up  beam  without  filleted  joints  (Fig.  46f),  the  work  to  the 
maximum  load  was  approximately  two-thirds  of  that  of  the  single-piece  beam.  On  the  other 
hand,  the  work  to  maximum  work  in  the  other  series  (fig.  46e)  was  20  per  cent  higher  than  for 
the  single-piece  beams.  Consideration  of  the  results  as  a  whole  indicate  that  this  type  of  beam, 
properly  glued,  will  compare  favorably  with  the  single-piece  construction. 


Note  12.  AIRCRAFT  DESIGN  DATA.  99 

Series  2  and  13  (fig.  46i) :  This  series  included  single-piece  spruce  beams  made  from  rejected 
material  compared  with  laminated  spruce  beams  made  from  similar  matched  material.  The 
laminated  material  gave  5  to  10  per  cent  lower  values  in  modulus  of  rupture  and  5  to  10  per 
cent  greater  values  in  work  to  maximum  load.  The  results  show  that  not  only  will  the  glue 
hold  satisfactorily  but  that  higher  values  would  not  be  secured  by  laminating  defective  material 
than  by  using  it  in  solid  form. 

Series  3  and  10  (fig.  46b) :  Series  3  is  made  from  one-eighth-inch  poplar  with  the  grain 
of  all  plies  longitudinal  compared  to  similar  material  and  construction  with  the  grain  of  the 
center  ply  vertical. 

Series  10  consists  of  beams  of  one-sixteenth-inch  poplar  laminations  with  vertical  joints. 

Three  types  were  made  up  as  follows:  (a)  The  grain  of  the  center  ply  vertical;  the  grain 
of  all  other  plies  horizontal.  (6)  The  grain  of  all  plies  having  a  slope  of  one  in  five  from  the 
horizontal,  the  slope  in  adjacent  plies  being  in  opposite  directions,  (c)  The  grain  of  the  six 
center  plies  having  a  slope  of  one  in  five  from  the  horizontal,  the  slope  in  adjacent  plies  being 
in  opposite  directions;  the  grain  of  all  other  plies  (namely  the  flange  plies)  being  horizontal. 

The  tests  showed  (1)  a  5  to  10  per  cent  reduction  in  the  mechanical  properties  where  the 
grain  of  the  center  ply  was  vertical,  with  no  reduction  made  in  the  thickness  of  the  web  due 
to  using  this  form  of  construction;  (2)  a  reduction  of  approximately  20  per  cent  in  total  load 
and  stiffness  where  a  slope  of  one  in  five  was  used  in  alternate  directions  in  adjacent  lami- 
nations throughout  the  whole  beam;  (3)  a  reduction  midway  between  the  foregoing  where 
a  slope  of  one  in  five  in  alternate  directions  was  used  only  in  the  web. 

The  conclusions  to  be  drawn  from  this  series  are  that  if  cross-grained  material  must  be 
used,  better  results  would  be  secured  by  laminating  and  placing  the  grain  of  adjacent  lamina- 
tions in  opposite  directions  than  to  use  solid  beams  of  similar  material,  but  that  it  would  not 
be  possible  to  secure  a  strength  equivalent  to  beams  of  satisfactory  grain  throughout. 

Series  4  (fig.  46g) :  A  plywood  web  with  Douglas  fir  flanges  was  used  in  this  series,  and 
included  beams  with  the  grain  in  the  outside  plies  of  the  web  longitudinal,  vertical,  and  at 
45  degrees.  Hide  glue  was  used  in  making  the  beams,  and  failures  of  the  glued  joints  developed 
in  the  tests  presumably  due  to  faulty  control  in  the  application  of  the  glue.  These  tests  are 
being  repeated,  using  casein  glue. 

Series  5  and  6  (figs.  46g  and  h) :  These  series  included  spruce  flanges  with  plywood  webs. 
The  face  plies  were  one- thirty-second  inch  with  vertical  gram,  while  the  thickness  of  the  core 
was  varied  from  one-eighth  to  one-sixteenth  inch.  The  results  indicate  the  desirability  of 
making  a  web  of  this  construction  somewhat  thicker  than  required  for  shear  stresses  only. 

Series  7  (fig.  46c) :  An  acceptable  grade  of  spruce  with  plywood  sides  was  used  in  these 
tests.     Four  thicknesses  of  plywood  were  used,  as  follows: 
Outside  plies  -^  inch,  core  J  inch. 
Outside  plies  ^  inch,  core  i  inch. 
Outside  plies  -^  inch,  core  y&  inch. 
Outside  plies  y-J^  inch,  core  ^  inch. 

This  type  of  beam  gave  very  satisfactory  results,  but  the  very  thin  plywood  proved  entirely 
inadequate.  The  results  indicate  that  plywood  with  a  one-sixteenth-inch  core  and  one-thirty- 
second-inch  faces  would  be  suficient  and  that  possibly  a  lighter  construction  would  prove 
satisfactory. 

Series  18  (fig.  46d) :  This  series  included  beams  made  up  of  one-sixteenth-inch  poplar 
veneer  with  the  center  ply  of  the  web  vertical.  The  results  were  satisfactory  and  showed 
that  the  glue  held  sufficiently  to  develop  the  strength  of  the  section.  This  type  of  beam,  how- 
ever, would  probably  be  increased  in  weight  about  10  per  cent  above  that  of  a  solid  beam  of 
similar  material  due  to  the  large  quantity  of  glue  which  would  be  required. 


100  AIRCRAFT  DESIGN  DATA.  Note  12. 

Series  11  and  19  (fig.  46j):  This  series  used  white  pine,  with  one-half  of  each  beam  of 
quarter-sawed  and  the  other  half  of  plain-sawed  material  and  with  moisture  content  5  per 
cent  higher  in  one-half  of  the  beam  than  in  the  other.  It  is  planned  to  subject  different  beams 
from  this  series  to  varying  conditions  of  humidity  in  order  to  determine  the  effect  of  such  con- 
ditions where  the  grain  of  the  two  faces  of  the  beam  are  of  a  different  character  and  in  differ- 
ent directions.  The  greater  part  of  this  series  has  not  yet  been  run,  but  the  variations  in 
results  not  due  to  the  gluing  indicate  that  greater  defects  can  not  be  allowed  in  either  piece 
than  are  now  allowed  in  solid  beams. 

GENERAL    CONCLUSIONS. 

In  general,  practically  all  types  of  beams  so  far  tested  have  given  values  commensurate 
with  what  might  be  expected  of  the  section  under  test.  In  other  words,  the  tests  have  shown 
that  waterproof  glue  properly  applied  enables  the  full  value  of  the  section  to  be  developed. 

Since  the  success  of  the  laminated  type  of  construction  is  primarily  dependent  upon  the 
efficiency  of  the  glue,  it  is  of  the  utmost  importance  that  means  be  provided  to  insure  the 
satisfactory  supervision  of  the  technique  of  gluing. 

The  types  of  beam  illustrated  in  figure  46e  and  f  and  in  figure  46c  seems  to  offer  the  most 
immediate  opportunity  for  effectively  increasing  production  from  the  class  of  material  now  on 
hand  and  being  received  by  the  airplane  manufacturers.  Since  in  the  types  indicated  in  figure 
46e  and  f  spiral  grain  material  can  be  used  in  the  webs,  these  types  would  have  the  particular 
advantage  of  permitting  utilization  of  material  now  rejected.  The  tests  thus  far  made  indicate 
that  these  beams  properly  made  are  no  more  variable  in  their  strength  properties  than  solid 
beams. 

All  of  the  beams  of  the  foregoing  series  were  made  under  laboratory  conditions.  In  order 
to  determine  just  what  might  be  expected  under  factory  conditions,  several  hundred  of  the 
types  shown  in  figure  46c  and  e  were  ordered  from  various  aircraft  manufacturers  and  tested. 
The  results  of  these  tests,  while  not  yet  completely  analyzed,  show  that,  with  proper  super- 
vision, it  is  possible  for  the  average  aircraft  manufacturer  to  produce  satisfactory  built-up 
beams.  They  also  show,  however,  that  the  need  for  thorough,  intelligent  supervision  is 
imperative. 

In  addition  to  these  series,  numerous  miscellaneous  types  of  beams  have  been  tested. 
Several  of  these  types  were  similar  to  the  types  which  have  become  more  or  less  standard, 
while  others  may  be  considered  freak  designs.  So  far  none  of  these  freak  designs  have  shown 
up  satisfactorily.  Several  of  the  designs  had  some  form  of  plywood  in  the  flanges.  In  no 
instance  have  beams  of  this  type  proven  as  strong  as  beams  with  solid  flanges  or  flanges  in  which 
all  the  grain  was  parallel  to  the  longitudinal  axis.  Figure  47  shows  various  types  of  wing  beam 
construction  which  have  been  used  in  machines  or  approved  for  use. 

BEAM    SPLICES. 

Until  the  present  year  the  matter  of  beam  splices  had  not  received  a  great  deal  of  attention. 
There  were  in  use,  and  embodied  in  specifications,  many  different  kinds  of  splices,  some  of 
which  were  obviously  very  inefficient.  The  growing  shortage  of  full-length  material  made  the 
matter  of  increasing  importance,  and  several  series  of  tests  were  run  both  in  this  country  and 
in  Great  Britain. 

The  following  report  is  based  on  tests  on  about  150  spliced  beams  and  150  unspliced  beams, 
each  spliced  beam  being  matched  to  an  unspliced  one  by  being  cut  alongside  of  it  out  of  the 
same  plank.  The  beams  were  all  Douglas  fir,  kiln  dried,  and  of  good  quality,  If  by  2f  inches 
in  cross  section,  and  the  splices  were  made  up  by  hand,  using  certified  hide  glue.  The  dowels 


Note  12. 


AIRCRAFT  DESIGN  DATA 


101 


Si    approved 
appro  wet 
Army  approved 


Sritisri  approved. 


Slrmy 


5.Y.A 
(A/f£rnafe  rou f 


CUV  TUNNEL  GOTM 


22 


m 


<^>SS? 


w 


G&f  /frets* 

(O/d). 


SOPW/T 

( Upper  on/y) 


S.C.5. 
HALBE'/rsrADT 


1 


Wide.  &h 
( IV//A 


3r/S/3rt  approved 
Outer  /ami'ncLf-/'bns 
faof  f/afije.  fhrcJrrtesi 


Fig.  47.— Typical  built-up  wing  spars. 


102 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


were  also  of  Douglas  fir.  In  no  cases  were  clamps  or  tape  used  to  reinforce  the  splices  ;  neither 
were  any  of  the  splices  bolted.  The  beams  were  all  tested  over  a  60-inch  span  under  third- 
point  loading,  thus  producing  uniform  bending  moment,  without  shear,  in  the  central  third 
of  the  span,  in  which  the  splices  were  all  located.  In  order  to  eliminate  as  many  variables  as 
possible,  the  efficiency  of  each  splice  was  calculated  in  per  cent  of  the  strength  of  the  unspliced 
beam  matched  with  it.  The  efficiencies  thus  obtained  were  then  averaged  for  each  type  of 
splice. 

Table  14  presents  in  condensed  form  the  data  secured  and  shows  the  average,  maximum, 
and  minimum  efficiencies  of  each  of  the  nine  types  tested.  A  number  of  these  types  were 
selected  for  test,  not  because  it  was  thought  that  they  would  develop  high  efficiencies  but 
because  they  had  already  been  used  or  included  in  some  specification. 


TABLE  14.  —  Strength  of  wing  learn  splices  —  spars,  If  by 

in  diameter. 


inches  in  cross  section;  dowels,  %  inch 


Wing  beam  splice  No  

i 

2 

3 

4 

6 

7 

g 

g 

Length  of  splice,  inches 

16  25 

11  00 

8  125 

5  50 

8  50 

16  25 

11  0 

8  125 

16  25 

Slope  of  splice  

1  in  10 

1  in  4 

1  in  10 

1  in  4 

1  in  10 

1  in  4 

1  in  10 

1  in  10 

Glued  area,  square  inches  .  .     .   . 

44  8 

18  43 

22  4 

9  22 

*23  40 

44  80 

18  43 

22  40 

85  60 

Minimum  efficiency    .   ... 

49  5 

17  5 

77  7 

39  1 

74  1 

74  9 

19  6 

72  0 

50  0 

Maximum  efficiency  

88.0 

53  2 

105  5 

86  5 

91  0 

107  0 

61  4 

123  5 

100  7 

Average  efficiency  

73  0 

34  0 

90  0 

66  7 

81  0 

86  4 

38  7 

100  0 

75  9 

*  Does  not  include  two  end  areas,  2x (2.75X0.406),  2.23  square  inches. 

The  conclusions  drawn  from  the  tests  are  as  follows: 

(1)  A  laminated  beam  spliced  in  one  lamination  is  stronger  than  a  solid  beam  spliced  with 
the  same  type  and  slope. 

(2)  Dowels  add  to  the  strength  of  splices  from  10  to  20  per  cent  on  the  average  for  the 
spliced  beams  tested. 

(3)  Plain  scarf  joints,  with  the  plane  of  the  scarf  vertical,  are  the  most  satisfactory  from 
all  points  of  view.     Dowels  or  bolts  provide  a  great  deal  of  residual  strength  in  case  of  glue 
failure,  while  adding  to  the  maximum  strength  as  well. 

(4)  In  general,  a  slope  of  scarf  of  one  in  ten  will  provide  a  satisfactory  joint  in  either  solid 
or  laminated  beams. 

It  is  very  interesting  to  note  that  the  British  have  arrived  at  practically  the  same  conclu- 
sions and  that  the  standard  British  splice  has  a  slope  of  one  in  nine,  with  dowels  or  bolts  and 
dowels. 


Note  12.  AIRCRAFT  DESIGN  DATA.  103 


STRUTS. 

The  discussion  and  conclusions  presented  in  the  following  paragraphs  are  based  upon 
strength  tests  conducted  on  about  400  struts  of  various  types,  some  of  which  were  made  of 
accepted  material  and  others  of  material  rejected  by  airplane  inspectors  for  one  reason  or  another. 
Among  the  principal  objects  of  these  tests  are  the  following: 

(a )  To  check  the  individual  designs  and  the  factors  of  safety  developed. 

(&)  To  determine  the  variability  of  the  material. 

(c)   To  study  the  effect  of  spiral  grain  and  other  defects  upon  the  properties  of  the 

finished  struts  and  to  develop  methods  of  inspection. 
Tests  have  been  made  upon  the  following  kinds  of  struts: 
Standard  J-l  inners,  accepted  and  rejected,  spruce. 
Standard  J-l  outers,  accepted  and  rejected,  spruce. 
Standard  J-l  center,  accepted  and  rejected,  spruce. 
DH-4  inners,  accepted  and  rejected,  spruce  and  fir. 
DH-4  outers,  accepted  and  rejected,  spruce  and  fir. 
F5-L  outers,  accepted,  spruce  (laminated,  1\  by  6f  inches). 

All  of  these  except  the  F5-L  struts  were  solid.  The  F5-L  struts  are  laminateji,  with 
three  laminations,  of  which  the  center  one  is  lightened  by  means  of  two  oblong  lightening  holes. 

METHODS    OF   TEST. 

The  following  kinds  of  test  were  made: 

(1)  Standard-screw   testing  machine,  used  for  making  column  tests  on  struts  with  the 
regular  end  fittings  supplied  by  the  manufacturer.     Slow,  uniform  speed  of  compression.     A 
number  of  these  struts  were  tested  up  to  the  maximum  load  repeatedly  without  any  injury. 

(2)  Standard-screw  testing  machine,  used  for  making  column  tests  on  struts,  with  special 
knife-edge  and  fittings,  which  provided  practically  perfect  "pin  ends."     Slow,  uniform  speed  of 
compression.     Many  of  the  struts  tested  repeatedly  to  maximum  load  without  injury. 

(3)  Dead-load  tests  on  struts  carried  nearly  to  the  maximum  load. 

(4)  Special  tests  in  hand  machines  designed  for  use  in  the  inspection  of  struts.     These 
machines  show  the  maximum  load  direct  or  allow  it  to  be  calculated  from  the  stiffness  in  bending. 

The  results  secured  are  presented  according  to  groups  of  struts  as  tested,  and  the  conclu- 
sions drawn  are  presented  at  the  end  of  the  discussion  for  each  group. 

TESTS    ON    STANDARD   J-l    STRUTS. 

The  first  series  tested  consisted  of  60  J-l  struts,  outers,  inners,  and  centers,  all  spruce. 
These  were  accepted  stock  and  were  tested  principally  to  check  the  designs  and  determine  the 
quality  of  the  spruce.  The  following  general  conclusions  were  drawn: 

(1)  The  quality  of  the  spruce  was  satisfactory,  except  that  10  struts  had  a  specific  gravity 
less  than  0.36. 

(2)  The  struts  were  all  slender  enough  to  enable  the  maximum  load  to  be  determined  with- 
out injury  to  the  strut.     In  fact  it  was  found  possible  to  load  the  struts  repeatedly  to  maximum 
load  without  injury. 

(3)  It  was  found  that  the  ball-and-socket  joints  provided  by  the  manufacturer  offered  some 
resistance  to  the  free  deflection  of  the  struts.     This  resistance  would  probably  not  be  present 
in  actual  flight,  due  to  vibration.     The  knife-edge  fittings  were  found  to  obviate  this  source 
of  error  and  were  adopted  as  the  standard  fitting  for  future  tests. 


104 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


The  loads  sustained  by  the  various  classes  are  as  follows: 


Minimum. 

Maximum. 

Average. 

Front  outers  

935 

1,510 

1,203 

Front  inners          

1,620 

2,980 

2,325 

.Rear  outers.                 

830 

1,505 

1,148 

Rear  inners  •  

1,610 

2,965 

2,067 

The  average  moisture  content  for  the  outers  was  8.2  per  cent  and  for  the  inners,  8.3  per 
cent. 

In  order  to  form  a  basis  for  comparing  the  variations  in  the  individual  struts  with  normal 
variations  in  the  spruce  itself,  an  analysis  of  the  stiffness  of  500  specimens  of  spruce  was  made, 
and  it  was  found  that  the  average  variation  of  the  individual  moduli  of  elasticity  from  the 
average  of  them  all  was  15  per  cent.  This  average  variation  was  secured  as  follows:  The  dif- 
ference between  each  individual  modulus  of  elasticity  and  the  average  modulus  was  expressed 
in  per  cent  of  the  latter,  and  these  percentage  differences  were  then  averaged  to  secure  the 
average  variation. 

The  average  variation  from  the  average  strengths  for  the  struts  compares  favorably  with 
this  figure  of  15  per  cent  and  is  tabulated  by  strut  classes: 

Per  cent. 

Outers. 13 

Inners 16 

Centers , 12 

fill  I    nil'// 

The  individual  variation  of  the  maximum  and  minimum  from  the  average  strengths  is 
as  follows,  again  by  classes  of  struts : 

Outers:  Per  cent. 

Minimum 30 

Maximum 30 

Inners: 

Minimum..... '/IVi1./!1:. 27 

Maximum fIa«kp.W.  A!; 40 

Centers: 

Minimum 34 

Maximum . .  14 

In  general,  the  struts  followed  Euler's  law  as  well  as  could  be  expected,  except  that  the 
ideal  load  deflection  curve,  OABC,  figure  48,  was  modified  in  the  actual  tests  to  a  curve  more 
nearly  represented  by  ODBC.  This  was  in  all  probability  due  to  unavoidable  eccentricity  of 
fittings  and  loading.  According  to  the  Euler  theory  the  elastic  curve  of  a  slender  column  is 
a  sine  curve.  The  actual  curve,  as  determined  by  direct  measurement,  approaches  very  near 
to  the  theoretical  curve  of  Euler. 

TESTS   ON   KEJECTED   J-l    STRUTS. 

This  group  of  struts,  spruce  outers  and  inners,  was  rejected  by  Government  inspectors, 
and  tested  primarily  to  determine  the  effect  of  defects  upon  the  strength  of  the  struts  and  to 
study  means  of  inspection.  Standard  methods  of  test  were  followed.  The  general  conclu- 
sions drawn  are  as  follows : 

(1)  As  a  group,  these  struts  were  not  as  good  as  the  40  accepted  struts  previously  tested. 
A  larger  portion  of  the  rejects  broke  suddenly  and  a  larger  proportion  broke  without  prelimi- 
nary compression  failure. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


105 


(2)  Forty-one  of  the  rejected  struts  appeared  to  the  laboratory  staff  making  the  tests 
as  satisfactory  regarding  both  direction  of  grain  and  specific  gravity.     With  one  exception, 
the  weakest  of  these  41  was  as  strong  as  the  weakest  of  the  40  accepted  struts  previously 
tested.     Further,  the  average  of  these  41  rejected  struts  was  nearly  as  good  as  that  of  the  40 
accepted  ones. 

(3)  There  were  18  struts  whose  diagonal  or  spiral  grain  was  between  1  in  15  and  1  in  20. 
Of  these,  16  compared  favorably  with  the  41  discussed  in  the  two  preceding  paragraphs. 

(4)  A  total  of  57  (16  plus  41)  of  the  100  rejected  struts  compared  favorably  with  the  40 
accepted  struts  previously  tested. 

(5)  It  is  possible  to  segregate  the  acceptable  struts  from  lots  of  rejected  struts  by  means 
of  simple  strength  tests  if  the  passing  values  are  appropriately  chosen  from  preceding  labo- 
ratory tests  on  struts  like  those  in  question. 

(6)  The  limiting  grain  may  safely  be  reduced  to  1  in  15  without  causing  a  reduction  in 
the  factor  of  safety,  provided  that  strength  tests  and  appropriate  passing  values  are  imposed. 
Such  a  plan  of  inspection  by  test  would  undoubtedly  increase  the  quality  and  percentage  of 
acceptance. 


Load 


D 


O 


Fig.  48 — Load  deflection  curves  for  slender  struts. 
TESTS   ON   STANDARD  DE   HAVILLAND   STRUTS. 

The  purpose  of  the  tests  was,  in  general,  to  check  the  design  calculations  and  afford  a 
direct  comparison  between  spruce  and  Douglas  fir  when  used  as  struts.  Half  of  the  struts 
were  tested  in  the  machine  in  the  usual  manner  and  the  other  half  were  tested  in  a  special 
dead-load  apparatus.  A  summary  of  the  results  follows: 

(1)  With  the  exception  of  one  strut,  a  spruce  stick  notably  below  specification  both  as  to 
spiral  grain  and  density,  all  the  struts  developed  maximum  loads  greater  than  that  for  which 
they  were  designed. 

(2)  The  weakest  of  the  fir  struts  was  notably  low  in  density,  but  still  it  was  considerably 
stronger  than  the  calculated  load. 

(3)  There  was  practically  no  difference  in  the  average  strengths  of  the  spruce  and  the  fir 
struts;  but  there  was  wider  variation  between  the  minimum  and  maximum  values  for  spruce 
than  for  fir.     Without  exception,  the  spruce  struts  were  lighter  than  the  lightest  fir  strut. 
For  unit  weight  (of  strut)  the  spruce  struts  were  17|  per  cent  stronger  on  the  average  than 
the  fir. 


[tape  sdi  bfifl  ,7io-)DJ8l  ad$  ni  alaai  noiioaqeai  Qfiiii/oi  o)  aid  ism  fttoH 


106  AIRCRAFT  DESIGN  DATA.  Note  12. 

(4)  In  the  dead-weight  test  all  of  the  struts,  with  one  exception,  were  stable;  that  is,  if 
deflected  by  a  side  push  (when  under  the  weight  of  3,200  pounds  chosen  for  the  test,  which 
was  just  under  the  crippling  load  for  the  weakest  strut),  they  would  come  back  upon  removal 
of  the  push.     The  exception  was  the  weakest  strut,  which  was  unstable  at  a  dead-weight  of 
3,030  pounds. 

(5)  Notwithstanding  the  general  low  specific  gravity  of  the  fir  struts,  the  maximum  loads 
which  they  sustained  were  high,  and  it  would  seem  safe  to  reduce  the  limit  from  0.47  to  0.45 
for  struts  of  the  same  size  as  spruce  and  to  use  fir  interchangeably  with  spruce. 

TESTS    ON    REJECTED    DE    HAVILLAND    STRUTS. 

These  tests  were  primarily  made  in  connection  with  the  development  of  strut- testing 
machines  and  inspection  by  actual  test.  There  were  70  spruce  and  70  Douglas  fir  struts,  all 
rejected  by  Government  inspectors  for  one  reason  or  another.  One  hundred  had  been  rejected 
for  spiral  grain  and  the  other  40  for  miscellaneous  defects,  which,  under  actual  test,  did  not 
influence  the  failures  at  all.  The  results  of  these  tests  confirmed  the  conclusions  drawn  from 
previous  tests,  both  as  to  the  need  and  practicability  of  a  strength  specification  and  test,  and 
the  limits  of  slope  of  grain  and  specific  gravity  for  Douglas  fir  already  mentioned.  In  addition, 
careful  study  was  made  of  the  variation  of  spiral  grain  along  the  length  of  the  strut  and  its 
effect  upon  the  maximum  load,  and  as  a  result  of  this  study  the  conclusion  has  been  reached 
that  for  struts  of  uniform  cross  section,  like  the  D-H  struts,  the  most  severe  requirements 
for  straightness  of  grain  should  be  limited  to  the  middle  third  and  to  the  tapered  ends  and  that 
the  requirements  for  the  balance  of  the  strut  can  be  more  lenient. 

The  final  recommendation  concerning  the  slope  of  grain  is  that,  assuming  the  determination 
of  the  maximum  load  for  each  strut  and  no  reduction  in  the  factor  of  safety,  the  steepest  slope 
allowed  in  the  center  third  and  in  the  tapered  ends  be  1  in  15  and  that  the  passing  load  for 
struts  with  a  slope  between  1  in  15  and  1  in  20  be  set  higher  than  for  straighter-grained  struts. 
Struts  with  a  slope  between  1  in  15  and  1  in  20  at  the  center  third  and  at  the  tapered  ends 
and  showing  the  larger  load  specified  for  them  are  to  be  allowed  a  slope  of  1  in  12  for  the  remainder 
of  the  strut;  also,  struts  with  straighter  grain  than  1  in  20,  which  also  show  the  larger  load 
specified  for  struts  with  steeper  slope,  may  have  a  slope  of  1  in  12  outside  the  middle  third 
and  the  tapered  ends;  but  struts  having  a  grain  straighter  than  1  in  20  in  the  middle  third 
and  in  the  tapered  ends  and  which  meet  the  lower  load  requirements  specified  for  them,  but 
do  not  meet  the  higher  load  specified  for  the.  struts  with  the  steeper  slope,  may  be  allowed  to 
have  grain  with  a  slope  of  1  in  15  or  straighter  in  the  remainder.  The  requirement  for  greater 
load  in  the  case  of  the  steeper  slopes  is  put  in  to  insure  against  possible  greater  variability  in 
shock  resistance  of  this  material. 

TESTS   ON    STANDARD   F5-L    STRUTS. 

The  main  purpose  of  these  tests  was  to  determine  whether  or  not  they  fall  in  the  class  of 
slender  struts  and  can  be  loaded  to  their  maximum  loads  without  injury.  These  struts  are 
built  up  of  three  laminations  each,  the  center  laminations  being  lightened  by  two  oblong  light- 
ening holes.  They  are  2|  by  6|  by  102  inches.  It  was  found  that  they  could  be  tested  up  to 
their  maximum  load  without  injury,  and  it  was  also  found  possible  to  calculate  the  maximum 
loads  by  means  of  stiffness  determinations  based  upon  simple  bending  tests.  The  details  of 
these  methods  will  be  described  in  the  following  paragraphs : 

TWO    NONINJURIOUS   TEST   METHODS    FOR    INSPECTING    STRUTS. 

Two  noninjurious  methods  of  test  for  determining  the  ultimate  strength  of  interplane  struts 
have  been  developed  as  a  result  of  the  series  of  tests  which  have  been  described  in  the  preceding 
pages.  Both  methods  are  applicable  to  routine  inspection  tests  in  the  factory,  and  the  equip- 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


107 


ment  needed  is  simple  and  cheap.     Both  methods  are  applicable  to  slender  struts  (all  the  struts 
so  far  have  fallen  in  this  class). 

In  order  to  determine  the  limiting  slenderness  ratio,  - ,  governing  the  use  of  these  two 

r 

methods  for  solid  spruce  and  Douglas  fir  struts,  tests  were  made  upon  three  spruce  and  three 

fir  struts,  as  follows:  They  were  first  tested  full  length,  -  about  165,  and  were  then  successively 

r 

shortened  to  —  ratios  of  140,  120,  100,  90,  and  80,  and  tested  at  each  length  by  both  methods. 

As  a  result  of  these  tests  the  conclusion  is  reached  that  for  spruce  the  limiting  slenderness  ratio 
is  about  100  and  for  Douglas  fir  about  90. 

Three  types  of  machine  have  been  built  and  tried  out  satisfactorily.  In  the  first  two  types 
the  strut  is  actually  loaded  up  to  the  maximum  load.  In  the  third  type  the  modulus  of  elas- 
ticity is  determined  by  means  of  a  simple  beam  test  well  within  the  elastic  limit  and  the  maximum 
load  calculated  by  a  simple  conversion  formula. 


Fig.  49.— Homemade  strut-testing  machine,  first  design. 

The  first  machine  (fig.  49)  employs  the  lever  principle  and  is  especially  suitable  for  larger 
strut  loads,  say  over  5,000  pounds.  A  (fig.  49)  is  a  strut  in  place  for  testing;  B  is  a  base  rigidly 
fastened  to  the  top  of  table  C;  it  affords  support  for  one  end  of  the  strut  and  also  for  the  pulling 
screw  D.  E  is  a  lever,  by  means  of  which  the  pull  (multiplied)  is  brought  to  bear  on  the  strut 
as  strut  load.  F  is  a  knife-edge  fulcrum;  and  G  a  spring  dynamometer.  H  and  I  are  supports 
for  pulling  rod  and  fulcrum  rod,  respectively.  J-J  are  the  stops  at  either  side  of  the  middle 
of  the  strut  to  limit  excessive  deflection  of  the  strut  through  careless  operation.  The  dial  K  is 
not  a  part  of  the  machine  for  making  the  proposed  acceptance  tests.  It  was  used  for  measuring 
strut  deflections  in  another  investigation.  The  dynamometer  (John  Chatillon  &  Sons,  of  New 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


York)  is  of  1,500  pounds  capacity.     It  is  graduated  in  25-pound  intervals,  and  5  pounds  can  be 
estimated  easily.     The  pulling  rig  is  an  ordinary  carpenter  bench  vise  screw,  handle,  etc. ;  the 

screw  has  eight  threads  to  the  inch. 

0 

The  second  machine  (fig.  50)  is  of  the  direct-pull  type  without  multiplying  lever,  especially 
suitable  for  the  smaller  strut  loads,  say  under  5,000  pounds.     It  consists  of  a  long  shallow  box, 


Fig.  50. — Homemade  strut-testing  machine,  second  design. 

into  one  end  of  which  a  rigid  and  strong  frame  is  built;  AB  is  the  frame  mentioned;  C  is  a  strut 
in  place  for  test.  The  load  is  brought  upon  the  strut  by  the  headpiece  D.  The  load  is  applied 
by  means  of  the  handwheel  E  on  the  screw  F;  it  is  applied  through  the  spring  dynamometer  G 
and  pulling  rods  H  to  the  headpiece  D.  The  rods  extend  freely  through  the  piece  B.  They  are 
supported  at  their  ends  by  the  headpiece  D  and  the  part  I,  both  on  castors  which  track  on  the 
floor  of  the  box  when  the  machine  is  used  in  horizontal  position.  J  is  wood  block  encircling  the 


D 


Fig.  51. — Beam  machine  for'strut  testing. 

pulling  nut.  It  prevents  the  nut  from  turning  and  affords  attachment  for  the  dynamometer. 
Adjustment  for  different  strut  lengths  is  afforded  by  the  turnbuckles  K  and  the  distance  rods  L. 
The  third  machine  (fig.  51)  is  a  "beam  machine"  for  the  second  method  of  determining 
strut  strength.  A  is  the  strut  in  place  for  testing;  BB  are  I  beams  forming  the  base  of  the 
entire  appliance;  they  support  the  weighing  scale  C,  the  loading  screw  D,  and  one  end  of  the 
strut.  The  middle  deflections  of  the  strut  are  measured  by  means  of  the  usual  device,  a  thread 
stretched  between  two  points  on  the  strut  just  over  the  supports  and  a  suitable  vertical  scale 
just  behind  this  thread  and  fixed  to  the  strut  or  to  the  loading  block  E. 


Note  12.  AIRCRAFT  DESIGN  DATA. 


Discussion  of  noninjurious  test  methods. — Keference  has  been  made  to  a  simple  formula 
used  to  calculate  the  maximum  load  of  a  strut  from  a  smaller  load  and  the  corresponding  deflec- 
tion in  a  bending  test  (the  method  illustrated  in  fig.  51).  The  following  discussion  will  show 
how  this  formula  is  developed. 

Euler's  column  formula  seems  to  be  in  most  common  use  for  calculating  the  maximum 
strength  of  interplane  struts,  and  the  method  under  discussion  is  based  mainly  on  that  formula. 
It  is: 

_C/7r2EI  Q\ 

^       L2 

Where  Q  =  Total  crushing  strength  of  column  in  pounds. 

C  =  A  coefficient  depending  on  the  character  of  the  end  bearings  (free  or  fixed). 

E  =  Modulus  of  elasticity  in  pounds  per  square  inch. 

I  =  Moment  of  inertia. 

L  =  Length  of  column  between  bearings  in  inches. 

The  deflection  in  a  strut  supported  flatwise  near  its  ends  and  then  subjected  to  a  cross- 
bending  load,  such  that  the  strut  (as  a  beam)  is  not  overstrained,  is  given  by  the  formula: 

,-^PF  (2) 

El 

Where  d  =  Deflection  at  center  in  inches. 

K = A  coefficient  depending  on  loading  and  manner  of  support  of  the  strut  as  a  beam. 
P  =  Any  moderate  (beam)  load,  not  overstraining  the  beam,  in  pounds. 
Z  =  Span  in  the  beam  test  in  inches. 
E  =  Modulus  of  elasticity  in  pounds  per  square  inch. 
I  =  Moment  of  inertia. 

For  any  given  strut  equations  (1)  and  (2)  may  be  equated  by  solving  for  El  in  both  cases, 
thus: 


-"0?"    d 

Solving  for  Q  gives  the  formula: 


For  struts  on  knife-edges  supports  C=l.  Struts  (on  ball-and-socket  supports,  pin  sup- 
ports, and  the  like)  in  flying  airplanes  are  subjected  to  vibration  which  breaks  down  the  friction 
at  the  supports  and  makes  the  supports  equivalent  to  knife-edges.  Hence  it  seems  wise,  as  in 
practice,  to  calculate  the  ultimate  strength  of  airplane  struts  as  though  knife-edge  supported; 
that  is,  with  0  =  1.  In  regard  to  the  most  suitable  kind  of  loading  of  the  strut  as  a  beam,  only 
center  and  third  point  were  considered;  others  were  regarded  as  impractical.  By  actual  trial 
of  12  struts  it  was  found,  contrary  to  expectation,  that  center  loading  gave  the  better  results; 
accordingly,  that  loading  was  finally  decided  upon.  For  such  loading  and  simple  nonrestraining 
supports,  K  =  1/48.  Hence  equation  (3)  becomes 

.206?     P  (  } 

^~    L2    *  d 

* 

which  is  the  final  form.     It  will  be  noted  that  P  and  d  (or  their  ratio)  are  the  only  quantities 

p 
for  which  test  must  be  made  in  order  to  furnish  the  value  of  Q  for  any  particular  strut.     -^  is 

the  center  load  per  inch  of  deflection;  it  is  therefore  a  measure  of  the  stiffness  of  the  strut. 


110 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


For  struts  not  uniform  in  cross  section  or  composition  the  Euler  (column)  formula  and  the 
beam  deflection  formula  still  hold.  Appropriate  mean  or  average  values  of  E  and  of  I  must, 
of  course,  be  used  in  each,  but  whether  or  not  these  average  values  in  the  column  formula  are 
respectively  equal  to  those  in  the  deflection  formula,  thus  permitting  their  cancellation  or  elimi- 
nation, can  not  be  answered  positively  for  all  nonuniform  struts.  It  is  believed  that  the  answer 
is  affirmative.  There  is  affirmative  evidence  from  tests  of  20  tapered  solid  struts  (10  outer 
and  10  inner  struts  for  the  J-l  airplane),  also  from  tests  on  5  built-up  struts  (5  pieces,  plywood 
covered);  that  is  to  say,  the  second  method  of  test,  based  on  formula  (4),  was  applied  to  these 
struts  and  very  good  results  were  obtained. 

Comparison  of  two  test  methods  by  actual  trials. — Thirty-five  struts  were  tested  by  the  beam 
method  and  for  comparison  by  the  column  method  also.  The  tests  by  the  beam  method  were 
made  with  the  struts  on  knife-edge  supports.  The  results  are  recorded  in  the  columns  marked 
Qj  (table  15).  The  results  by  the  column  method  are  recorded  in  columns  marked  Q2  (table  15). 
The  per  cent  differences  between  Qt  and  Q2  appear  in  the  following  columns.  They  are  decidedly 
small,  and  the  test  verification  of  the  theory  of  this  second  method  is  highly  satisfactory.  The 
table  includes  solid  struts  of  spruce  and  Douglas  fir,  both  of  uniform  and  tapered  section,  and 
struts  of  uniform  section  built  up  of  spruce  and  birch. 

TABLE  15. —  Maximum  or  crippling  loads  for  certain  struts  determined  by  measurement  in  column 

tests  and  by  calculation  from  cross-bending  tests. 
(a)  Solid  struts  uniform  in  section. 


No. 

Species. 

Qi 

Q, 

Qi-Qj 

Average  grain. 

Qi 

Z=52  inches. 

1=60  inches. 

1=52  inches. 

Z=60  inches. 

Spiral. 

Diagonal. 

DH-4  inners 

Gr-41 

Spruce  

Pounds. 
5,175 
6,350 
5,125 
4,375 
3,445 

2,075 
2,240 
2,560 
2,020 
2,460 

2,540 
1,800 
1,975 
1,950 
2,170 

1,450 
1,235 
1,060 
1,415 
1,390 

5,380 
6,420 
5,270 
4,310 
3,640 

2,040 
2,200 
2,520 
2,035 

2,485 

2,570 
1,750 
1,920 
1,920 
2,220 

1,  425 
1,195 
1,010 
1,355 
1,385 

5,390 
6,530 
5,530 
4,575 
3,645 

2,080 
2,180 
2,570 
2,060 
2,510 

2,510 
1,820 
1,945 
2,030 
2,200 

1,430 
1,200 
1,030 
1,385 
1,360 

Per  cent. 
-4.0 
-1.1 
-2.8 
+1.5 
-5.6 

+1.7 
+1.8 
+1.6 
-0.7 
-1.0 

-1.2 
+2.8 
+2.8 
+1.5 
-2.3 

+1.7 
+3.2 

+4.7 
+4.2 
+0.4 

Per  cent. 
-4.1 
-2.8 
-7.9 
-4.6 
-5.8 

-0.2 
+2.7 
-0.4 
-2.0 
-2.0 

+  1.2 
-1.1 
+1.5 
-4.1 
-1.4 

+1.4 
+2.8 
+2.8 
+2.1 
+2.2 

65 
65 
80 
14 
25 

30 
15 
39 
18 
16 

95 
50 
80 
60 
100 

80 
95 
21 
80 
95 

G-42         

do  

G-56                    .   .   .. 

....  do  

G-57            . 

do  

G-64            

do  

DH-4  outers: 
G-70  

Fir... 

G-74                

.   .     do  

G-76      

do  

G-79                

.   ...do  

G-80  

do  

J-l  inners: 
D-l..  

Spruce  

D-13  .  . 

do  

D-14     

do  

D-17  

do  

D-2  

Fir  

J-l  outers: 
D-19  

Spruce  

D-20   

....  do  

D-21  

do  

D-7  

do  

D-8   

.   .     do    

Average  

2.4 

2.7 

Qt=Max.  load  as  measured  in  column-bending  test. 
Q2=Max.  load  as  calculated  from  cross-bending  test. 
n  _  2PZ3 


D=Deflection  at  load  P  in  cross  bending. 
Z=Span  in  cross  bending. 
L=Effective  length  in  column  bending. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


Ill 


TABLE  15. — Maximum  or  crippling  loads  for  certain  struts  determined  by  measurement  in  column 

tests  and  by  calculation  from  cross-bending  tests — Continued. 

(6)  Solid  struts  tapered  (Span=Z=64  inches). 


No. 

Species. 

Qi 

Q2 

Qi-Qs 

Qi 

J-l 

inners  : 
D  1  

Spruce 

Pounds. 
2  275 

Pounds. 
2  450 

Per  cent. 
7  1 

D-13  

...     do 

1  700 

1  720 

-1  2 

D-14  

...     do 

1  790 

]   835 

—2  5 

D-17  •.  

.    .     do 

1  775 

1  750 

4-1  4 

D-19  

do 

1  400 

1  430 

—2  1 

D-2  

Fir 

2  030 

2  120 

—4  4 

J-l 

outers: 
D-20  

Spruce  ... 

1,165 

1,210 

—3  9 

D-21  

...     do 

1  000 

1  040 

-4  0 

D-7 

Fir 

1  300 

1  330 

-2  3 

D-8  

do 

1  315 

1  350 

—2  6 

Average  

3  2 

(c)  Built-up  struts,*  uniform  in  section  (span=Z=60  inches). 


No. 

Species. 

Qi 

Q, 

Qi-Qs 

Qi 

J-14  

( 

Pounds. 
4  250 

Pound*. 
4  160 

Per  cent. 
4-2  1 

.1-15  

J-16  

4,815 

4,710 

4-2  2 

j  17 

All  spruce  and  birch  

3  760 

3  600 

4-4  2 

J-18  

3  500 

3  540 

—1  1 

J-19              

3  425 

3  440 

—0  4 

Average  '  

2  0 

*  The  core  was  a  double  box  made  of  spruce;  it  was  covered  or  stream  lined  with  two-ply  spruce;  the  inner  ply  was  longitudinal,  about  one- 
eighth  inch  thick,  the  outer  circumferential,  about  one-thirty-second  inch  thick.    Other  dimensions  were  as  for  DH-4  inners. 

It  will  be  noted  that  many  of  the  struts  were  tested  on  two  spans.  One  span  was  practi- 
cally the  maximum  which  the  strut  afforded.  The  two  spans  were  tried  out  to  ascertain  whether 
choice  of  span  is  important.  As  expected,  the  choice  was  unimportant  with  struts  of  uniform 
cross  sections,  but  with  tapered  struts  the  longest  span  gave  best  results.  Several  struts  were 

tested  twice  on  the  same  span.     The  second  time  turned  over — that  is,  the  side  which  was  the 

p 

upper  in  the  first  test  was  the  lower  in  the  second.     The  values  of  -v  in  the  two  tests  were  prac- 
tically alike  in  each  case. 

A  high  degree  of  skill  is  not  necessary  in  using  the  cross-bending  test  for  inspecting  struts, 
but  for  good  results  care  should  be  taken  about  details.  Both  ends  of  the  strut  should  be  sup- 
ported in  such  a  way  that  bending  can  occur  without  the  ends  slipping  on  the  supports.  The 
supports  should  be  such  that  there  is  no  doubt  where  the  points  of  support  are,  because  the 
exact  value  of  span  is  required  in  formula  (4).  The  bending  load  P  is  relatively  small  com- 
pared with  the  maximum  (100  to  400  pounds  for  struts  so  far  tested).  Hence,  a  weighing  appa- 
ratus correct  to  1  or  2  pounds  should  be  provided.  The  deflection  should  be  read  with  reference 

to  points  on  the  strut  immediately  over  the  support  and  not  on  the  machine.     For  best  results 
p 

a  single  value  of  -7  should  not  be  relied  upon.     Good  practice  is  to  read  loads  and  deflections 
a 

. 


112  AIRCRAFT  DESIGN  DATA.  Note  12. 


-|-v 

for  a  load  deflection  graph.     The  mean  straight  line  gives  the  best  value  of  -r  for  use  in  the 

formula.     Of  course,  the  loadings  should  not  be  carried  to  the  elastic  limit.     In  the  tests  of  J-l 
and  DH-4  struts  deflections  up  to  one-half  inch  were  used.     This  was  really  more  than  necessary. 

p 
All  that  is  needed  is  enough  of  the  (straight)  load  deflection  graph  to  be  certain  of  its  slope,  -5  • 

MISCELLANEOUS    STEUT   TESTS. 

Tests  of  struts  stream  lined  with  plywood. — Seven  struts  of  two  distinct  designs  were  tested 
as  square-ended  columns  and  compared  directly  with  solid  spruce  struts  of  the  same  gross  area 
and  solid  spruce  struts  of  the  same  weight,  also  tested  as  square-ended  columns.  The  sections 
of  the  built-up  struts  are  shown  in  figure  52a  and  b.  The  test  length  was  5  feet.  As  was  to 
be  expected,  the  design  shown  in  figure  52a  did  not  develop  satisfactory  strength,  and  after 
testing  four  struts  the  design  shown  in  figure  52b  was  developed  and  three  struts  made  up, 
using,  respectively,  birch,  soft  maple,  and  red  gum  plywood.  These  struts  developed  about 
double  the  strength  of  the  other  type,  and  appear  to  be  rather  well  balanced  (as  square-ended 
columns),  since  one  of  them  failed  by  shearing  of  the  spruce  web. 

The  plywood  struts  were  naturally  larger  than  solid  spruce  struts  of  the  same  strength 
and  shape,  although  lighter,  and  consequently  would  create  greater  wind  resistance  or  drift. 

(7)  W 


Fig.  52.— Spruce  and  plywood  struts. 

In  order  to  reach  an  equitable  basis  of  comparison  it  was  necessary  to  consider  both  weight 
and  drift.  Assuming  an  air  speed  of  80  feet  per  second  and  that  1  pound  of  resistance  is 
equivalent  to  6  pounds  of  weight,  the  equivalent  weight  of  the  plywood  struts  was  calculated 
to  be  91  per  cent  of  that  of  the  solid  struts  (of  the  same  strength  and  shape)  at  this  speed. 

Naturally  at  higher  speeds  the  advantage  of  the  plywood  struts  is  correspondingly  less, 
disappearing  entirely  long  before  present  maximum  speeds  are  reached.  The  average  weight 
of  the  plywood  struts  was  3.91  pounds  and  the  average  actual  load  sustained  was  9,700  pounds 
(as  square-ended  columns). 

Tests  on  struts  covered  with  bakelized  canvas. — Tests  were  made  on  24  spruce  struts,  more 
or  less  cross  grained,  and  covered  with  bakelized  canvas  (micarta).  The  external  dimen- 
sions of  all  the  struts  were  alike,  but  half  were  covered  with  two  layers  of  canvas  and  the  other 
half  with  four  layers;  the  former  having,  therefore,  more  wood  in  them  than  the  latter.  All 
the  struts  were  tested  in  column  bending  for  maximum  load  without  injuring  them.  All  were 
subsequently  tested  to  failure,  16  with  the  canvas  partially  or  wholly  removed. 

Since  the  modulus  of  elasticity  of  bakelized  canvas  is  lower  and  its  specific  gravity  much 
higher  than  that  of  spruce,  one  would  expect  this  material  to  be  a  poor  substitute  for  spruce 
in  struts,  so  far  as  total  strength  and  strength  per  unit  weight  of  strut  is  concerned.  All  tests 
made  verify  this  expectation,  but  the  canvas  covering  improved  the  quality  of  the  defective 
spruce  struts  in  one  respect,  namely,  the  capacity  of  the  strut  to  withstand  severe  shock.  This 
conclusion  is  based  on  the  fact  that  the  deflection  at  failure  for  eight  covered  struts  was  con- 
siderably greater  than  for  four  struts  stripped. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


113 


The  struts  covered  with  two  layers  of  canvas  were  stronger  than  those  with  four  because 
there  was  more  wood  in  them  and  they  were  much  stronger  per  unit  of  weight  than  the  latter. 

Struts  covered  with  canvas  were  but  little  stronger  than  the  same  struts  stripped  of  canvas. 
The  covered  ones  were  weaker  than  the  stripped  ones  per  unit  weight  of  strut.  Further,  it 
is  computed  that  the  canvas-covered  struts  were  weaker  than  spruce  struts  of  the  same  size 
would  have  been. 

Comparisons  with  40  J-l  struts  previously  tested  show  that  the  covered  struts  were  not 
as  high  in  total  strength  or  strength  per  unit  weight  as  the  plain  struts. 

Several  struts  had  the  outer  layer  of  canvas  removed  for  some  distance  from  the  ends, 
and  these  struts  so  stripped  were  to  all  intents  and  purposes  as  strong  as  they  were  originally. 

Effect  of  taper  on  the  strength  of  struts. — Tests  were  made  on  40  solid  struts  to  determine 
the  effect  of  taper.  These  struts  were  of  spruce  and  Douglas  fir.  Some  were  of  the  sizes  and 
shapes  corresponding  to  DH-4  inners  and  outers  and  the  others  of  the  sizes  and  shapes  cor- 
responding to  the  central  sections  of  Standard  J-l  inners  and  outers.  (It  will  be  remembered 
that  the  J-l  struts  have  a  central  section  about  0.46  the  length  of  the  strut,  which  is  of  uni- 
form section,  the  taper  starting  at  the  ends  of  this  section  and  running  in  a  smooth  curve  to 
the  ends.)  These  40  struts  were  all  first  tested  for  maximum  load  while  of  uniform  section. 
They  were  all  tapered  to  the  geometrical  form  of  the  J-l  taper  and  again  tested  for  maximum 
load.  Finally  the  DH-4  struts  were  given  a  very  pronounced  taper  and  tested  a  third  time 
for  maximum  load.  The  results  of  the  series  of  tests  are  presented  in  condensed  form  in 
table  16. 

TABLE  16. — Effect  of  taper  on  the  strength  and  weight  of  struts. 


Change  due  to  first  taper. 

Change  due  to  second  taper. 

Lot  No. 

Number 
of  struts. 

A. 

B. 

c. 

A. 

B. 

C. 

Per  cent. 

Per  cent. 

Per  cent. 

Per  cent. 

Per  cent. 

Per  cent. 

1 

3 

-4.9 

+0.6 

+5.7 

-22.2 

-32.3 

-6.6 

2 

3 

-  4.4 

0.0 

+4.4 

-26.7 

-23.5 

-3.9 

3 

5 

-  3.7 

-1.5 

+2.1 

-17.8 

-20.3 

-3.6 

4 

5 

-  5.1 

-2.0 

+3.1 

-18.8 

-22.1 

-4.6 

5 

7 

—  8  1 

—3  1 

+4  8 

6 

5 

—  9  0 

—3  5 

+6  2 

7 

7 

—  9  7 

—2  4 

4-7  9 

8 

5 

-11.2 

-3.1 

+9.0 

A  represents  change  in  weight  due  to  taper. 

B  represents  change  in  maximum  load  due  to  taper. 

C  represents  change  in  maximum  load  per  unit  weight  of  strut,  due  to  taper. 

Lots  1  to  4  were  DH-4  struts,  and  lots  5  to  8  of  J-l  size. 

The  maximum  load  per  unit  weight  was  increased  by  the  first  taper  from  a  minimum  of 
2.1  per  cent  for  lot  3  to  a  maximum  of  9  per  cent  for  lot  8.  The  weighted  average  increase  was 
5.5  per  cent. 

It  will  be  noted  that  the  second  taper  reduced  the  strength  weight  ratio  as  well  as  the 
maximum  load. 

DESIGN   AND   MANUFACTURE    OF   BUILT-UP   STRUTS. 

The  following  general  discussion  is  based  upon  the  results  of  several  hundred  thousand  tests 
on  wood  in  various  forms,  as  well  as  upon  the  experience  gained  in  the  design,  manufacture, 
and  test  of  struts  of  various  types.  While  much  of  the  discussion  is  quite  obvious,  it  is 
believed  to  be  pertinent. 

98257— 19— No.  12 8 


114  AIRCRAFT  DESIGN  DATA.  Note  12. 

Built-up  struts  possess  a  number  of  advantages  and  disadvantages  as  compared  to  the 
solid  one-piece  construction,  some  of  which  are  as  follows: 
Advantages : 

Use  of  small  pieces  of  material. 

More  effective  distribution  of  material. 
(a)  By  routing. 
(6)  By  using  materials  of  different  density. 

Possibility  of  using  defective  material. 

Complete  failure  may  not  occur  with  failure  of  one  lamina. 
Disadvantages: 

Greater  warping  or  bowing  if  pieces  are  not  rightly  selected  and  well  manufactured. 

Greater  difficulty  in  manufacture. 

Greater  time  required  for  manufacture. 

One  of  the  mam  advantages  of  built-up  struts  is  the  possible  use  of  smaller  dimens'on 
material  with  its  corresponding  lower  cost  and  greater  availability.  It  is  further  a  matter  of 
common  observation  that  many  of  the  larger  pieces  which  contain  defects  such  as  to  make  them 
unsatisfactory  for  use  as  a  single  unit  would  yield  smaller  pieces  free  from  defects  and  suitable 
for  built-up  construction.  The  material  near  the  center  of  a  solid  strut  contributes  but  little 
in  proportion  to  its  weight  to  the  maximum  load  the  strut  will  carry.  Struts  lightened  by  routing 
at  the  center,  therefore,  have  the  advantage  of  a  greater  strength-weight  ratio  than  a  solid 
strut.  Enough  material  at  the  major  axis  of  symmetry  is,  of  course,  necessary  to  carry  the 
shear,  which  is  gfeatest  along  this  axis  and  near  the  ends  of  the  strut.  A  built-up  strut  lends 
itself  readily  to  routing  or  lightening  at  the  center. 

The  taper  of  solid  struts  is  likewise  meant  to  accomplish  a  reduction  in  weight.  Weight 
reduction  with  a  minimum  reduction  in  strength,  however,  can  probably  be  most  effectively 
obtained  through  routing  in  built-up  construction.  This,  however,  is  more  feasible  with  struts 
of  larger  dimension,  and  probably,  all  things  considered,  should  not  be  undertaken  on  struts 
whose  minor  axis  is  less  than  If  inches.  It  is  common  practice  in  built-up  struts  lightened  in 
this  manner  to  discontinue  the  routing  at  regular  intervals,  thus  leaving  a  solid  cross  section 
at  these  given  points. 

Use  of  materials  of  different  density. — It  may  be  shown  that  a  metal  column  with  proper 
distribution  of  material  will  theoretically  withstand  a  load  two  or  three  times  greater  than  a 
solid  wooden  section  of  the  same  total  weight,  length,  and  section  boundary.  This  is  based 
on  the  assumption  that  no  local  buckling  takes  place.  With  thin  metal  walls  this  assumption 
would,  of  course,  not  be  strictly  true,  as  buckling  actually  does  occur.  The  conclusion  is  valid, 
however,  that  the  denser  material,  with  its  greater  stiffness,  may  be  desirable  for  struts  and  is 
most  effective  when  distributed  at  the  greatest  possible  distance  from  the  neutral  axis.  This 
points  to  the  possible  advantages  of  a  combined  wood  and  metal  strut  and  demonstrates  in 
built-up  wooden  struts,  especially  the  larger  sizes,  that  the  use  of  denser  species  for  the  outer 
portions,  with  a  lighter  species  for  a  core,  would  furnish  a  possible  efficient  combination.  The 
use  of  a  combination  of  species  of  wood  of  different  density,  however,  would  not  be  desirable 
in  solid  built-up  struts  of  small  size,  and  if  used  in  the  larger  sizes  would  require  special  con- 
struction to  distribute  stresses  resulting  from  unequal  changes  in  dimension  and  unequal  stiff- 
ness, as  will  be  considered  later. 

Tests  on  combined  metal  and  wood  struts  are  now  under  investigation,  and  while  very 
encouraging  results  have  been  obtained  additional  work  along  this  line  will  be  necessary  before 
definite  recommendations  can  be  made  for  production  consideration. 


Note  12.  AIRCRAFT  DESIGN  DATA.  115 

Possibility  of  using  defective  material. — But  little  data  is  available  on  the  effect  of  defects 
such  as  spiral  or  diagonal  grain  in  the  individual  pieces  on  the  strength  of  built-up  struts.  In 
connection  with  the  use  of  spiral  grain  material  for  struts,  however,  it  may  be  noted  that  the 
modulus  of  elasticity  is  not  as  greatly  reduced  by  this  defect  as  are  the  other  mechanical  prop- 
erties, and  therefore  the  maximum  load  in  struts  which  is  largely  dependent  on  the  stiffness 
may  not  be  greatly  reduced  with  slopes  of  grain  as  great  as  1  in  15.  In  built-up  struts  con- 
taining but  one  glued  surface  parallel  to  the  major  axis  the  limitations  of  defective  material 
should  be  maintained  up  to  the  standard  required  for  one-piece  construction.  Large  struts, 
however,  may  be  composed  of  three  (or  more)  sections,  as  shown  in  figure  54.  The  center  section, 
containing  the  major  axis  of  symmetry,  receives  little  other  than  shear  stress.  It  is  probable 
that  a  greater  tolerance  of  grain  could  be  permitted  here  than  in  the  outer  laminations  or  in 
one-piece  construction.  Tests  to  secure  information  on  this  point  are  necessary  and  are  under 
consideration. 

Possibility  of  warping  or  bowing. — One  difficulty  frequently  encountered  on  the  manu- 
facture of  built-up  struts  is  the  tendency  to  warp  or  bow.  Practically  all  wood  contains  inter- 
nal stresses  to  a  greater  or  lesser  extent,  and  failure  to  take  into  consideration  the  factors  which 
influence  these  stresses  contributes  largely  to  the  trouble  mentioned.  As  is  well  known,  wood 
changes  dimensions  at  right  angles  to  the  grain  to  a  considerable  extent  with  change  in 
moistuie  content.  Unequal  changes  in  the  widths  of  various  laminations  causes  severe  stress 
in  the  glued  joints  and  may  even  cause  failure.  Among  the  important  factors  which  cause 
unequal  changes  in  dimensions  in  the  different  laminations  are : 

(a)  The  use  of  plain-sawed  and  quarter-sawed  laminations  in  the  same  strut. 

(6)  The  use  of  laminations  that  differ  in  density. 

(c)   The  use  of  laminations  that  differ  in  moisture  content. 

(a)  In  connection  with  the  use  of  plain-sawed  and  quarter-sawed  material  it  may  be 
noted  that  the  shrinkage  of  Sitka  spruce  in  a  radial  direction  is  only  about  jix- tenths  of  that 
in  a  tangential  direction.  For  a  given  change  in  moisture,  it  will  theiefore  be  seen  that  a 
plain-sawed  board  would  normally  undergo  a  greater  change  in  dimension  than  would  quarter- 
sawed  material.  In  built-up  construction  the  best  results  would  therefore  be  expected  with 
quai  ter-sawed  material,  as  shown  in  sections  1-a  and  1-b,  figures  53  and  54.  The  use  of  both 
plain  and  quarter  sawed  material  in  the  same  built-up  part  should  be  avoided. 

(6)  Another  factor  which  may  influence  the  warping  of  built-up  struts  is  the  density  of 
material  in  adjacent  laminations.  It  has  been  shown  that  in  general  the  shrinkage  of  wood 
varies  directly  as  the  density,  and  light  pieces  would  therefore,  as  a  rule,  retain  their  shape 
better  than  denser  ones.  The  adjacent  laminations  should  be  made  of  pieces  of  approxi- 
mately the  same  density  to  give  the  best  results,  as  otherwise  considerable  stress  may  be  intro- 
duced along  the  glued  joints,  due  to  the  tendency  of  the  various  laminations  to  change 
dimensions  unequally. 

(c)  Differences  in  the  moisture  content  of  the  various  laminations  at  the  time  of  manu- 
facture may  also  contribute  to  the  warping  of  built-up  struts  or  other  parts.  Since  wood 
shrinks  with  change  of  moisture  content  and  since  all  material  stored  or  used  under  similar 
conditions  will  ultimately  assume  approximately  the  same  moisture  content,  it  follows  that 
differences  in  moisture  content  at  the  time  of  gluing  will  cause  unequal  changes  in  dimen- 
sions which  introduce  stresses  in  the  glued  surface.  The  fact  that  all  material  used  in  a  given 
laminated  member  comes  from  the  same  stock  does  not  necessarily  insure  against  differences 
in  moisture  content  between  individual  pieces.  The  wide  range  in  the  rate  of  drying  of  indi- 
vidual pieces,  the  difference  in  drying  between  quarter-sawed  and  plain-sawed  lumber,  as  well 
•q  siii  Tk»  iooBs  6ib  oj  as  oMisiiav/*  *i  tr»Y9woji  <,»*«{>  .-iia  goibnod  oi 


116 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


as  the  fact  that  heavy  pieces  usually  dry  more  slowly  than  lighter  ones,  contribute  to  the  dif- 
ferences of  moisture  content  which  may  be  found  at  any  time  in  a  given  stock.  The  position 
of  material  in  a  pile  while  air  seasoning  or  in  the  kiln  while  being  dried  may  also  influence  the 
rate  of  drying  and  consequently  the  difference  in  moisture  content  between  individual  pieces 
at  a  given  time. 

The  manufacture  of  built-up  struts  with  proper  attention  to  the  various  factors  which 
may  affect  the  quality  of  the  product  as  outlined  in  the  preceding  discussion  would  be  more 
difficult  than  the  manufacture  of  single-piece  members.  The  time  required  for  inspection 


2a  3a.  -a.  /6 

Fig.  53. — Sections  of  built-up  struts,  two  and  four  piece  construction. 


/a. 


20. 


A 


46 


Fig.  54.— Sections  of  built-up  struts,  three  and  six  piece  construction. 


would  be  increased  on  account  of  the  greater  number  of  pieces  involved  and  because  of  the 
matching  required.  The  gluing  would  also  be  an  additional  item  to  be  considered  in 
manufacture. 

The  additional  work  involved  in  the  proper  manufacture  of  laminated  struts  would  prob- 
ably have  a  tendency  to  reduce  production,  or  at  least  would  require  greater  facilities  and 
more  labor  for  a  given  output — particularly  for  struts  of  smaller  sizes.  These  considerations 
would  tend  to  offset  the  lower  cost  resulting  from  the  more  complete  utilization  of  the  small 
pieces. 

Static  and  impact  bending  tests  made  on  Sitka  spruce  and  a  few  other  species  have  shown 
that  the  position  of  the  growth  rings  with  respect  to  the  faces  of  the  test  pieces  does  not  influ- 
ence the  bending  strength.  No  data,  however,  is  available  as  to  the  effect  of  the  position  of 


Note  12.  AIRCRAFT  DESIGN  DATA.  117 

growth  rings  on  the  strength  of  struts,  although  it  is  expeceted  that  some  data  along  this  line 
will  be  secured  in  the  near  future.  From  data  available  at  present  the  position  of  growth 
rings  in  a  built-up  strut  would  be  expected  to  affect  physical  properties,  such  as  the  ability 
to  retain  shape  rather  than  strength.  It  is  desirable  in  built-up  members  that  the  construc- 
tion be  such  as  to  reduce  the  stresses  to  a  minimum.  This  involves  the  use  of  material  of 
approximately  the  same  rate  of  growth,  density,  moisture  content,  and  direction  of  growth 
rings  in  the  cross  section. 

CONCLUSIONS. 

1.  The  manufacture  of  built-up  struts  with  a  minor  axis  of  If  inches  or  less  is  not  recom- 
mended. 

2.  (a)  To  secure  the  best  results,  the  laminations  of  built-up  strut  should  be  approximately 
of  the  same  moisture  content,  density,  rate  of  growth,  and,  in  general,  except  in  cases  of  special 
design,  of  the  same  species. 

(6)  The  construction  of  stream-line  struts  should  be  symmetrical  about  the  major  axis. 
It  may  be  noted  that  symmetry  and  consequent  balance  of  internal  stresses  can  in  some  cases 
be  secured  without  conformity  to  the  exact  requirements  under  (a}  above. 

3.  Figures  53  and  54  show  recommended  sections  of  built-up  struts. 

(a)  Sections  1-a  and  1-b  in  both  figures  53  and  54  would  be  expected  to  give  the  greatest 
freedom  from  internal  stresses  and  consequent  warping. 

(6)  In  figure  53  but  little  difference  in  ability  to  retain  shape  would  be  expected  between 
sections  2-a,  3-a,  and  4-a,  and  also  between  2-b,  3-b,  and  4-b. 

(cZ)  There  are  a  great  number  of  possible  combinations  of  material  with  different  combina- 
tions of  growth  rings,  and  it  is  quite  possible  that  other  combinations  giving  modification  of 
types  shown  should  also  prove  satisfactory. 

4.  In  types  such  as  1-b,  2-b,  3-b,  and  4-b  in  both  figures  53  and  54  it  is  desirable  but 
not  essential  that  the  edge  joints  come  under  the  end  fittings. 

5.  The  edge  joints  as  shown  in  types  1-b,  2-b,  3-b,  and  4-b  in  figures  53  and  54  should  be 
staggered,  preferably  about  1  inch. 

6.  The  taping  of  built-up  struts  hides  the  glued  surfaces  from  inspection  and,  as  it  does 
not  add  to  the  strength,  seems  unnecessary. 

7.  The  use  of  waterproof  glue  for  built-up  struts  is  recommended. 

8.  There  is  reason  to  believe  that  the  construction  of  solid  and  routed  built-up  struts  can 
be  improved  over  present  practice  and  over  that  here  shown  so  as  to  more  effectively  relieve 
the  internal  stresses  which  tend  to  produce  warping.     It  should  be  remembered,  therefore,  that 
while  the  information  here  presented  is  based  on  the  most  complete  data  now  available  on 
built-up  struts  the  subject  is  one  which  has  been  but  little  studied  and  great  improvements 
may  consequently  be  expected. 


118 


AIECRAFT  DESIGN  DATA. 


Note  12 


Figure  55  shows  various  types  of  strut  construction  which  have  been  used  in  machines  or 
proposed  for  use. 


atnemavcnqmi  1 


Fig.  55. — Typical  built-up  strut  sections. 


Note  12. 


AIRCEAFT  DESIGN  DATA. 


119 


WING  RIBS. 

The  construction  and  loading  of  wing  ribs  is  of  such  a  nature  that  it  is  practically  impos- 
sible to  calculate,  with  any  reasonable  degree  of  accuracy,  the  actual  strength  of  any  particular 
design.  Further,  it  is  quite  impossible  to  determine  without  actual  test  the  relative  efficiency 
and  strength  of  the  various  elements  of  the  rib.  As  a  result  of  these  conditions  it  has  been 
found  necessary  to  develop  a  number  of  types  through  test.  Some  of  the  types  which  have 
been  used  or  proposed  for  use  are  shown  in  figure  56.  A  number  of  these  types  have  been 
tested,  and  several  of  them  were  developed  as  a  result  of  the  experiments. 


7.  P/ywooct  we£>  ;  osa./  //t 


<3.  Wwooct  we£>;  ova/ 

Fig.  56. — Typical  wing-rib  designs. 


120 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


CD  dD^ 


/O 


/S    Se/n/- /russ  (S.  £~. 


/6.    Se/n/- Truss  (/lancf/ey 

Fig.  56.— Typical  wing-rib  designs 


OOC 


.  s 

; 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


121 


//-uss  ••  a//  wooct      La.f/-/c.e. 


Ill 


23. 


rru.ss  •'  wood  aS7c(  /nera./  (Mowe  fy^e-) 


Detat/  of  mocftf teat/on  of 
A/o.  2&/fr  tv/i/c/i  encf/ess  basias 
arc  u..st£t tfi jo/&de  of  wft 

or/oops 


Fig.  56.— Typical  wing-rib  designs. 

The  two  outstanding  conclusions  from  the  tests  are:  (1)  The  type  of  rib  most  suitable  for 
small  and  medium  chords,  from  the  standpoint  of  the  strength  weight  ratio  combined  with 
manufacturing  ease,  is  the  plywood  web  type,  with  oval  and  circular  openings  (fig.  56,  case  8) 
and  with  vertical  grain  in  the  outer  plies  of  the  web. 

(2)  The  type  of  rib  most  suitable  for  large  chords  is  the  full  truss  type.  This  has  the 
greatest  strength- weight  ratio  of  all  types,  and  the  manufacturing  difficulties  are  not  over- 
whelmingly large  in  the  case  of  large  ribs. 


122 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Minor  conclusions  will  be  found  in  the  discussions  of  the  development  of  the  various 
individual  types. 

The  method  of  test  is  briefly  as  follows : 

The  ribs  are  mounted  in  a  testing  machine  specially  equipped  to  apply  the  load  to  the 
ribs  at  a  number  of  points  and  the  testing  head  is  run  down  at  a  slow  uniform  speed  until  failure 


i        i  —  ...  ^^^••^^^^••••^^••••••i^^^^^^MMa^^^^K— . 

Fig.  57. — Apparatus  for  testing  small  wing  ribs 


occurs.  In  the  case  of  small  ribs  the  load  is  applied  at  8  points,  as  shown  in  figure  57.  With 
the  larger  ribs  16-point  loading  is  used  (fig.  58).  During  the  test  the  travel  of  the  testing  head 
is  recorded  at  the  various  loads,  and  for  some  of  the  ribs  the  deformation  at  a  number  of  points 
along  the  rib  is  measured.  Figure  59  shows  the  relation  between  the  total  load  in  pounds  and 


Fig.  58. — Apparatus  for  testing  large  wing  ribs. 


the  travel  of  the  testing  head  in  inches.     The  strengthening  and  stiffening  accomplished  by 
judicious  reinforcement  are  clearly  shown. 

The  load  distribution  used  in  the  first  series  of  tests  is  shown  in  figure  60.     Later  a  triangular 
distribution  was  adopted,  in  which  the  apex  of  the  triangle  is  one-fourth  of  the  chord  from  the 


leading  edge  (fig.  68). 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


123 


25O 


Fig.  59.—  Wing  rib  load—  deformation  curves:  DH-4  ribs. 


Fig.  60.— Low-speed  load  distribution  used  in  wing  rib  tests. 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


TESTS    ON   DH-4    WING   KIBS. 


The  first  ribs  upon  which  development  work  was  undertaken  were  some  DH-4  ribs  sub- 
mitted by  one  of  the  manufacturers.  The  original  design  is  No.  1,  figure  61.  It  was  found 
that  this  rib,  which  has  a  plywood  web,  was  lightened  out  too  much  near  the  spars,  and  the 


Fig.  61.— Tests  on  DH-4  wing  ribs. 


Rib. 
No. 

Designation. 

Description. 

Number 
of  tests. 

Net  weight 
of  rib, 
ounces. 
W. 

Average 
total  load 
sustained, 
pounds, 
P. 

Ratio  of 
strength 
to  weight, 
P 
W 

Faces. 

Core. 

1 

2 

3 

4 
5 

Dayton-  Wright  

^-u^h  birch  

T^-inch  yellow  pop- 
lar. 
do  

4 

2 
3 

3 
2 

3 
5 

-      3 
5 

4 

4 

3 
3 
3 

2 

7.71 

5.23 

5.58 

5.26 
5.59 

5.85 
5.06 

5.64 
6.37 
6.12 
5.46 

5.20 
5.61 
5.52 

5.40 

136 

232 
243 

274 
232 

243 

253 

266 
297 
300 

274 

288 
325 
337 

346 

17.7 

44.4 
43.5 

52.1 
41.5 

41.5 
50.0 

47.2 
46.6 
49.0 
50.2 

55.4 
57.9 
61.0 

64.0 

Improved  original  

•j-^g-inch  maple  

^V-inch  yellow  pop- 
lar, 
•j^-inch  Spanish  ce- 
dar, 
yfopinch  maple  

•yfr-inch  basswood  .  .  . 

do  

•jVinch  Spanish  ce- 
dar, 
•^-inch  yellow  pop- 
lar. 
do  

Complete  truss  

^Vinch  Spanish  ce- 
dar. 
T^-inch  birch  

TV-inch  Spanish  ce- 
dar. 
TVinch  yellow  pop- 
lar. 
iV-inch  yellow  pop- 
lar. 
•jJj-inch  yellow  pop- 
lar, 
t^-inch  yellow  pop- 
lar. 
.....do  
do  

Semitruss  
Circular  opening  

q^-inch  basswood  *.  . 
....do  

^Vinch  yellow  pop- 
lar. 
..vdo  

^-inch  basswood  .  .  . 

•TTj-inch  birch  

•jJj-inch  yellow  pop- 
lar. 
•iVinch  Spanish  ce- 
dar. 

^y-inch  Spanish  ce- 
dar. 

*  Core  and  face  grain  run  parallel  and  perpendicular  to  diagonal  members. 


Note  12. 


AIECEAFT  DESIGN  DATA. 


125 


first  improvement  consisted  in  changing  the  shape  and  size  of  the  lightening  holes  and  inci- 
dentally reducing  the  weight  by  making  the  face  veneer  much  lighter.  The  improvement  in 
strength  is  shown  in  the  last  column  of  the  table.  Further  development  work  led  through  the 
semitruss  and  full  truss  (plywood)  designs  to  the  design  which  was  finally  decided  upon  as  the 
best  obtainable  (No.  5).  This  rib  is  shown  drawn  to  scale  in  figure  62. 

Several  other  types  of  DH-4  ribs  were  submitted  for  test,  among  them  being  several 
similar  to  case  13,  figure  56.  These  were  found  to  be  very  weak  indeed,  but  stiffening  and 
strengthening  by  means  of  wires,  case  24,  figure  56,  produced  a  marked  improvement.  In  fact, 
one  rib  developed  as  much  as  42  pounds  per  ounce  of  weight. 

Conclusions  drawn  from  these  tests,  which  included  150  ribs,  are  as  follows:    • 

(1)  Plywood  webs  are  superior  to  single-piece  webs  in  strength,  even  if  the  latter  are 
reinforced  with  vertical  strips  glued  and  nailed  in  position. 

(2)  Plywood  webs  with  the  face  grain  vertical  are  superior  to  plywood  webs  having  the 
face  grain  longitudinal. 

(3)  Nails  in  the  cap  strips  are  practically  useless  in  so  far  as  contributing  to  the  strength 
of  the  rib  is  concerned. 

(4)  Cap  strips  should  be  fastened  rigidly  to  the  spars. 

(5)  The  circular-opening  type  of  rib  is  superior  to  the  other  types  tested. 

(6)  For  the  size  of  rib  tested  a  core  of  one-sixteenth  yellow  poplar  or  Spanish  cedar  veneer 
with  longitudinal  grain  is  satisfactory.     If  high-density  wood,  like  birch,  is  used  for  face  veneer, 
the  thickness  should  be  from  one-sixtieth  to  one-seventieth  inch,  while  if  low-density  face  veneer, 
such  as  yellow  poplar,  is  to  be  used,  a  thickness  of  one-fortieth  to  one-fiftieth  inch  is  required. 

(7)  Low-density  face  veneer  is  superior  from  the  standpoint  of  manufacture  of  the  ply- 
wood, and  also  gives  somewhat  greater  stiffness  for  the  same  weight. 

(8)  Spruce  cap  strips  re  by  YJ  mcn  are  satisfactory.     They  should  be  grooved  and  well  glued. 

TESTS   ON    SE-5   WING    RIBS. 

Table  17  presents  the  test  data  on  a  number  of  SE-5  ribs  of  the  original  design  and 
of  the  design  developed  at  the  laboratory.  The  original  ribs  submitted  for  test  were  similar  to 
case  15,  figure  56,  and  consisted  of  22  pieces.  Under  low-speed  loading,  figure  60,  these  ribs 
developed  a  strength  of  25.3  pounds  per  ounce  of  weight,  and  under  high-speed  loading,  figure  68, 
the  average  strength  was  28.1  pounds  per  ounce  of  weight. 

TABLE  17. —  Tests  on  SE-5. wing  ribs. 

• 


Rib  number. 

Type  of  rib. 

Web  construction. 

Load  distribu- 
tion. 

Net 
weight 
of  rib. 
oz.,  W. 

Total 
load 
sus- 
tained. 
Ibs.,  P. 

P 
W 

Faces. 

Core. 

Average  of  1,  2,  6,  7  ... 
Average  of  11,  12,  13,  14 

Original 

Spruce  braces  and  g 
do  

truts  '.  . 

Low  speed..  . 
High  speed  .  . 
Low  speed  
High  speed  .  . 
Low  speed  .  .  . 

High  speed  .  . 
Low  speed  — 
High  speed  .  . 

6.67 
6.59 
6.17 
5.89 
4.61 

4.  60 
4.21 
4.23 

169 
185 
315 
291 
270 

249 
246 
275 
:;i  aiL 

25.3 
28.1 
51.0 
49.4 
58.5 

54.2 
58.5 
65.0 

do    

Plywood  No.  1 
do    

^5-inch  birch  

^-inch  basswood  .  . 
do  

A  v^rao-A  nf  8  Q  10 

do  

Average  of  19,  20,  21,  22 

Average  of  15,  16,  17,  18 
Average  of  23,  24,  25,  26 
Average  of  27,  28,  29,  30 

Plywood  No.  2 
do    '.  . 

^Vinch  Spanish  ce- 
dar. 
.....do  

Yf-inch  Spanish  ce- 
dar. 
do  

do  
do        

do  
do  

.....do  
do  

-viq  fit 
,78  bint  , 


Ribs  No.  1  to  22,  inclusive,  loose  in  spars,  bound  by  wire  wound  around  rib  at  spars. 

Ribs  No.  23  to  30,  inclusive,  were  glued  to  spars.    ;(  HJ{W  _  Q  iyn  l 

In  all  plywood  web  ribs  the  face  grain  was  vertical. 

Cap  strips  for  ribs  1,  2,  6,  7,  11,  12,  13,  and  14  were  A  by  *  inch  spruce. 

Cap  strips  for  ribs  3,  4,  5,  8,  9,  and  10  were  &  by  &  inch  spruce. 

Cap  strips  for  ribs  15  to  30,  inclusive,  were  J  by  •&  inch  spruce.    ]      .£  ] 


126 


AIECEAFT  DESIGN  DATA. 


Note  12. 


The  design  finally  developed  is  shown  in  detail  in  figure  63.  Several  types  were  made 
up,  using  different  species  and  thicknesses  of  veneer  in  the  web  plywood.  Of  these  the  ribs 
having  webs  composed  of  one-fortieth-inch  Spanish  cedar  faces  and  one-sixteenth-inch  Spanish 
cedar  core  proved  to  be  the  strongest  per  unit  of  weight.  The  strength  under  low-speed  loading 
was  58.5  pounds  per  ounce  of  weight,  and  under  high-speed  loading  a  strength  of  65  pounds 
per  ounce  of  weight  was  developed. 

Besides  being  much  stronger  and  lighter  than  the  original  ribs,  the  final  design  is  decidedly 
stiff  er. 

TESTS   ON   HS    WING   RIBS. 

The  original  HS  ribs  have  a  pine  web,  and  are  of  the  general  type  shown  in  case  4,  figure 
56.  The  final  design  is  of  the  plywood  web,  oval  and  circular  opening  type,  and  is  shown  in 
detail  in  figure  64.  Detailed  results  of  the  tests  are  presented  in  table  18.  Attention  is 
directed  to  the  cap  strips,  which  are  patterned  after  the  design  used  by  Fokker  in  his  recent 
biplane.  Better  cap-strip  fastening  is  secured  by  this  method  when  the  web  is  thin.  The 
basswood  faces  on  the  plywood  web  of  the  final  design  appear  to  be  somewhat  light,  and  it 
is  anticipated  that  better  results  would  be  secured  by  the  use  of  slightly  heavier  veneer. 

TABLE  18. —  Tests  on  HS-1L  wing  ribs. 


Type  of  construction. 

Load  distribution. 

Net  weight 
of  rib, 
ounces, 
W. 

Total  load 
sustained, 
pounds, 
P. 

Ratio  of 
strength 
to  weight. 
P 

w' 

Remarks. 

Present  construction  single-ply  pine 

Hi°h  speed 

16.80 

410 

£-inch,  single-ply  pine  web;  ^  by 

web. 
Do                

do  

17.28 

440 

f  inch  spruce  cap  strips;  1  by 
^  inch  stiff  eners  on  each  side  of 

Do                                

do  

16.31 

350 

web  between  openings. 

Average                       

do  

16.80 

400 

24 

Present  construction  single-ply  pine 

Low  speed 

16.  15 

458 

Construction  same  as  above. 

web. 
Do                                     .... 

do        .   ... 

16.31 

530 

do  

16.80 

590 

t<^<)  ^ 

Average      

do  

16.42 

526 

32 

Circular  opening  plywood  web 

High  speed 

10  96 

385 

Ply  -wood  web;  Tj-inch  basswood 

Do            

.  do  

10.81 

365 

faces;  J-inch  basswood  core;  -& 

Do 

do 

10  81 

310 

by  J  inch  spruce  cap  strip  on 

each  side  of  web;  grain  of  faces 
of  web  vertical. 

Average  

...do    

10  86 

353 

32 

Circular  opening  plywood  web 

Low  speed 

10  68 

490 

Construction  same  as  above. 

Do  .'  

do 

10  75 

600 

Do 

do      .. 

10  90 

500 

i  Ml       . 
Average  

do  

10.78 

530 

49 

TESTS   ON    F5-L    WING   RIBS. 

The  original  F5-L  ribs  were  of  the  general  type  of  the  HS  ribs,  case  4,  figure  56.  In 
developing  the  new  ribs,  it  was  thought  that  the  use  of  a  full  truss  type  rib  might  be  justified 
and,  therefore,  a  rib  of  this  type  was  designed  and  tested.  Further,  a  truss  type  with  ply- 
wood web  was  included  in  the  series.  The  three  designs  are  shown  in  figures  65,  66,  and  67, 
and  the  results  of  the  tests  upon  the  three  types  with  high-speed  loading  and  low-speed  load- 
ing are  shown  in  table  19.  Data  on  the  strength  of  the  original  design  are  also  included. 


Note  12. 


AIRCEAFT  DESIGN  DATA. 


127 


'Q, 

0, 


V 


128 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


j      £ 

D, 


foi 


.1 


.   i 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


129 


TABLE  19.—  Tests  of  F5-L  wing  ribs . 


Design  of  rib. 

. 

Load  distribution. 

Net  weight 
of  rib, 
ounces. 
W. 

Total 
load 
sustained, 
pounds. 
P. 

Ratio  of 
strength 
to  Weight. 
P 
W 

TJl                  J          '            1 

Plywood,  circular  opening  

High  speed  

15.5 

540 

...'..do  .    .    .         . 

do 

15  5 

485 

do  

do 

15.7 

400 

Average  1,2,  and  3  

15.6 

475 

31 

Plywood,  circular  opening  

Low  speed  

15.5 

592 

do  ,  

do 

15.7 

498 

do  

do    

15.5 

642 

, 
Average  4,  5,  and  6  

15.6 

577 

37 

Plywood  truss  

High  speed                .   . 

22.4 

508 

do      

.  .  do 

23.4 

533 

...     do      

..  do 

26.4 

670 

Average  7,8,  and  9  

24.0 

570 

23.  7 

Plvwood,  truss  

Low  speed  

23.0 

610 

do  

do  

23.8 

578 

do      

do      

23.0 

683 

Average  10  11,  and  12  

23.3 

624 

26  7 

Truss            

High  speed 

12.5 

580 

do                

do 

12.5 

505 

do                 

do 

12  3 

520 

Average  13   14  and  15  

12  4 

535 

43 

Truss            

Low  speed  

12.5 

665 

do         

do  

12.9 

710 

do               .-...-  

do  

12  6 

610 

Average  16   17,  and  18      

12  7 

662 

52 

Original  design                - 

High  speed 

22  1 

485 

do                

do  

22  1 

405 

do      

do  

21.0 

400 

do      

do  

21.  8 

435 

Average  19   20  21   and  22        

21  7 

431 

20 

Original  design     

Low  speed     

22  8 

593 

r\(\                                                                         

do 

23  4 

585 

.  do  

23  5 

550 

a  

do  

24.2 

590 

\verage  23  24   25,  and  26  

23  5 

579 

25 

It  will  be  seen  from  the  data  presented  that  the  full  truss  type,  figure  67,  developed  very 
much  greater  strength  per  unit  weight  than  either  of  the  other  types  and  that  the  plywood 
truss  type,  figure  66,  was  by  far  the  weakest  of  the  three.  Final  choice  between  the  full  truss 
type  and  the  plywood  web  type  must  be  determined  by  the  relative  importance  of  weight 
saving  and  cost  of  production. 
98257— 19— No.  12 9 


130 


AIRCKAFT  DESIGN  DATA. 


Note  12. 


TESTS    ON    15-FOOT    WING    BIBS. 

The  largest  ribs  so  far  tested  have  a  15-foot  chord  and  were  designed  for  a  machine  under 
contemplation  but  not  yet  built.  Three  general  types  of  rib  were  first  tested,  a  plywood  web 
circular  opening  type,  a  semitruss  type  with  reinforced  plywood  web,  and  a  full  truss  type 
with  vertical  compression  members  and  diagonal  tension  members  (Pratt  type).  A  glance  at 
table  20  shows  that  the  full  truss  was  far  superior  to  the  other  types  in  strength-weight  ratio. 
The  low-speed  load  distribution  used  is  shown  in  figure  60  and  the  high-speed  distribution  in 
figure  68.  The  full  truss  design  is  shown  in  detail  in  figure  69.  The  stiffness  of  this  design  is 
illustrated  in  figure  70,  which  shows  the  relation  between  the  travel  of  the  testing  head  and 
the  toal  load  in  pounds.  The  uniformity  in  the  properties  of  the  three  ribs  is  noteworthy. 
The  need  for  thorough  fastening  of  the  cap  strips  and  the  verticals  to  the  spars  is  emphasized. 

TABLE  20. —  Tests  on  15-foot  wing  ribs. 


No. 
of 
rib. 

Type  of  rib. 

Species  of  web. 

Load  distribution. 

Cap  strips. 

Net 
weight 
of  rib, 
pounds, 
W. 

Total 
load 
sus- 
tained, 
pounds, 
P. 

P 
\V 
w= 

Weight 
in 
ounces. 

Faces. 

Core. 

1 
2 

7 
8 

10 

9 
11 
12 
13 
14 

Circular  opening  .  .  . 
do 

^s-inch  birch., 
^y-inch  birch.. 

^5-inch  Spanish 
cedar, 
do      

Low  speed  
.     .  do  

J    by    J   inch 
spruce. 
.  do  

2.42 
2.28 

251 
318 

6.5 

8.7 

Average  values 

2.35 

285 

7.6 

Semitruss       

yVinch  birch.. 
^Vmch  birch.. 

T^-inch  Spanish 
cedar, 
do      

High  speed  .  .  . 
do  

}   by    I   inch 
spruce. 
do.  

2.92 

2.68 

286 
175 

6.1 
4.1 

do 

Average  values 

2.80 

231 

5.1 

Truss  

Spruce  compre 
web. 
do 

ssion  members  and 

High  speed  .  .  . 
..do... 

•&  by  f  inch 
spruce. 
do  

2.49 

2.41 
2.42 
2.48 
2.49 
2.44 

565 

672 
710 
707 
721 
690 

14.2 

17.4 
18.3 
17.8 
18.1 

17.7 

do                 

do 

do                    

do  

do  

do         

do  

do  

do  

do                 

..do      

do  

do  

do        

do  

do  

do  

Average  values* 

2.45 

700 

17.9 

^^_  *  (Rib  No.  10  culled  and  omitted.) 

After  this  series  of  tests  was  completed  it  .was  thought  desirable  to  develop  a  truss  type  of 
rib  which  did  not  depend  so  largely  upon  glue  for  the  security  of  the  fastenings,  and  so  a  rib 
of  the  Warren  type  was  designed  and  three  built  and  tested.  The  design  is  shown  in  figure  71 
and  table  21  presents  the  results  of  the  tests  and  also  the  results  of  the  previous  tests  on  the 
Pratt  type  for  comparison.  While  the  objects  aimed  at  were  attained,  it  was  at  the  sacrifice 
of  considerable  weight,  as  will  be  seen  from  an  inspection  of  the  table.  Tests  have  just  been 
completed  upon  a  number  of  modified  ribs  of  the  Warren  type.  These  ribs  showed  a  greater 
strength-weight  ratio  than  any  other  15-foot  ribs  tested  at  the  laboratory. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


131 


132 


AIRCRAFTgDESIGN  DATA. 


Note  12. 


700 


600 


v 

\ 

^ 


£00 


/OO 


O.2 


ae 


/.ff 


Fig.  70.  —  Wing  rib  load-deformation  curves:  Pratt  truss  type  ribs  for  15-foot  chord  machine. 


TABLE  21. — Tests  on  15-foot  wing  ribs. 
Pratt  truss  and  Warren  truss  type. 


No. 
of 
rib. 

Type  ol  rib. 

Construction. 

Cap  strips. 

Net  weight 
of  rib 
pounds. 
W. 

Total  load 
sustained, 
pounds. 
P. 

P 
W 

W=  weight 
in  ounces. 

q 

Pratt  truss  

Spruce  compression  members 

^  by  |  inch  spruce.  .  ,  

2.41 

672 

17.4 

11 

do 

and    birch   veneer   tension 
members, 
do      

..do.. 

2.42 

710 

18.3 

1? 

do 

do  

do  

2.48 

707 

17.8 

IS 

do 

do  

do  

2.49 

721 

18.1 

14 

do 

..  do  t... 

do  

2.44 

690 

17.7 

Average  values 

2.45 

700 

17.9 

15 

Warren  truss 

Plywood  members  

Spruce  channel  see  sketch  .... 

3.72 

770 

12.9 

16 

do 

do                     

do  

3.54 

855 

15.  1 

17 

do 

do  

do  

3.63 

830 

14.3 

Average  values 

3.63 

850 

14.  1 

Ribs  tested  with  high  speed  load  distribution. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


133 


134 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


In  addition  to  these  types  of  15-foot  rib  experiments  were  made  upon  three  other  types, 
as  follows: 

1.  Full  truss  type,  with  vertical  compression  members  and  diagonal  tension  members 
running  in  both  directions.     These  diagonal  members  consisted  of  two  birch  veneer  bands 
wrapped  continuously  around  the  whole  rib  from  end  to  end,  one  to  the  right  and  the  other  to 
the  left.     These  bands  passed  around  the  caps  at  the  panel  points.     These  ribs  developed  a 
strength  of  13  pounds  per  ounce  of  weight. 

2.  Full  truss  type,  similar  to  1,  except  that  the  veneer  bands,  instead  of  passing  around 
the  caps,  passed  between  the  caps  and  ends  of  the  verticals,  being  given  a  twist  at  this  point. 
The  strength  developed  was  9  pounds  per  ounce  of  weight. 

3.  Full  truss  type,  similar  to  1,  except  that  the  veneer  bands,  instead  of  passing  around 
the  caps,  were  cut  at  these  points  and  glued  to  the  sides  of  the  caps,  which  were  of  channel 
section.     This  type  developed  a  strength  of  13  pounds  per  ounce  of  weight  and  has  the  advantage 
of  greater  ease  of  assembly  than  types  1  and  2. 

It  is  to  be  noted  that  none  of  these  types  developed  as  great  strength  as  either  the  Pratt 

or  Warren  types. 

TESTS  ON  ELEVATOR  OR  AILERON  SPARS. 

Comparatively  little  is  known  about  the  behavior  of  wood  under  torsion.  This  has  not 
been  of  particular  importance  in  the  past,  but  the  proper  design  of  control  surface  spars  demands 
such  knowledge.  Mention  has  been  made,  under  Mechanical  and  Physical  Properties  of  Wood, 
of  a  few  torsion  tests  made  on  solid  specimens  of  spruce  and  ash.  A  few  tests  have  also  been 
made  on  hollow  dummy  control  spars  of  Sitka  spruce.  The  individual  results  of  the  tests  are 
given  in  table  22,  and  a  comparison  between  these  results  and  those  on  the  solid  specimens 
previously  mentioned  is  shown  in  table  23.  Details  of  the  test  specimens  will  be  found  in 
figure  72. 

TABLE  22. — Individual  results  of  torsion  tests  on  15  hollow  Sitka  spruce  elevator  spars. 


Specimen  No. 

Moisture, 
per  cent  of 
oven-dry 
weight 

Specific 
gravity 
(oven-dry 
weight 
and 
oven-dry 
volume). 

Shearing 
stress  at 
elastic 
limit 
(pounds 
per  square 
inch). 

Shearing 
stress  at 
maximum 
load 
(pounds 
per  square 
inch). 

Shearing 
modulus  of 
elasticity 
(pounds  per 
square  inch). 

Work  to 
elastic  limit 
(inch  pounds 
per  cubic 
inch). 

Work  to 
maximum 
load  (inch 
pounds 
per  cubic 
inch). 

1                 .                 

12.6 

0.44 

500 

1,000 

92,  100 

1.12 

7.1 

2   

15.0 

.48 

820 

1,370 

83,  300 

3.38 

15.5 

3                     

13.5 

.38 

950 

1,000 

79,  500 

4.75 

6.4 

4                            

12.8 

.48 

610 

780 

88,  900 

1.72 

5                                                

15.0 

.45 

930 

1,260 

76,100 

4.74 

11.4 

6       

14.2 

.51 

820 

1,170 

77,  700 

3.62 

10.5 

7                              

14.6 

.34 

710 

940 

55,  300 

3.84 

9.1 

8    

13.2 

:43 

910 

1,270 

75,900 

4.54 

13.8 

9       

14.8 

.50 

820 

1,070 

77,  800 

3.62 

8.0 

10                                     

13.6 

.47   ' 

820 

1,030 

83,  400 

3.38 

6.9 

11  

12.0 

.52 

740 

1,040 

73,  800 

3.06 

7.3 

13                                                . 

13.4 

.48 

1,400 

14               

15.2 

.37 

910 

1,350 

80,600 

4.27 

15.8 

15                                         

13.4 

.43 

840 

1,080 

71,900 

4.13 

9.5 

16 

14  3 

49 

970 

Average                      

13.8 

.455 

800 

1,110 

78,  200 

3.55 

10.11 

Note  12. 


AIRCRAFT  DESIGN  DATA. 


TABLE  23. — Summary  of  results  of  torsion  tests  on  hollow  Sitlca  spruce  elevator  spars  and  tests 

on  solid  circular  specimens. 


.;?-••  .  :•<•'•••;<;  y. 

:      •      •:        i!      ,',('>    V-     Ml  !. 

Tests  on  15 
hollow  elevator 
spars,  Sltka 
spruce  (1). 

Tests  on  15 
solid  circular 
specimens 
Sitka  spruce, 
(2). 

Ratio  of  (1) 
to  (2)  in  per 
cent. 

!     •     »  «.!  ;••;  •    '.ft., 

Moisture,  per  cent  of  oven-dry  weight.  1  

13.8 
0.46 
1  800 
1,110 
1  78,  200 
1  3.6 
1  10.1 

15.7 
0.  39 
1,090 
1,650 
72,  300 
4.4 
19.7 

88 
118 
73 
67 
108 
82 
51 

Specific  gravity,  based  on  oven-dry  weight  and  oven-dry  volume  

Shearing  stress  at  elastic  limit  (pounds  per  square  inch)  

Shearing  stress  at  maximum  load  (pounds  per  square  inch)                

Shearing  modulus  of  elasticity  (pounds  per  square  inch)                     

Work  to  elastic  limit  (inch  pounds  per  cubic  inch)    

Work  to  maximum  load  (inch  pounds  per  cubic  inch)                  

1  Based  on  13  tests. 


OT'" 

*'J! 

L                                  '•?"                                   . 

r 

. 

A  '"'  , 

/<?" 

^" 

T4 

**       1 

Tesf  spec/me/? 
are  3/?ot+fs? g/u<sa '//?  e/7c/s. 


s/oGC/mef? 


cross-secf/pr?  of  e/e 
S/OG/~  /-esfec/  //?  fors/o/? 


/?ff  /77gc/?//?e 


Defa/SofP/ag 


co/7/7ec 
Fig.  72. — Torsion  test  specimen. 

It  is  to  be  noted  that  80  per  cent  of  the  specimens  failed  at  or  near  the  spline  joint,  indi- 
cating that  the  joint  was  a  source  of  weakness  in  the  specimens. 

The  relation  between  specific  gravity  and  strength  in  shear  is  not  definite  enough  to  be 
used  as  a  basis  for  selection  of  material  to  withstand  shearing  stresses. 

These  tests,  as  well  as  torsion  tests  in  general,  are  subject  to  large  variations.  These  varia- 
tions are  probably  more  pronounced  in  hollow  spliced  construction  and  will  therefore  neces- 
sitate using  very  large  safety  factors  in  order  to  obtain  safe  working  stresses. 

In  addition  to  the  tests  already  mentioned,  a  few  tests  have  been  made  upon  hollow  spars 
with  a  hollow  wooden  core,  around  which  veneer  is  wrapped  in  right  and  left  spirals.  The 
indications  are  that  both  the  ultimate  strength  in  torsion  and  torsional  stiffness  can  be  doubled 
by  this  method  of  construction. 


136 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


TESTS   ON   AIRCRAFT    ENGINE    BEARERS. 

A  short  series  of  tests  was  made  to  determine  the  relative  merits  of  engine  bearers  built 
of  all  veneer  and  those  built  with  a  spruce  filler.  A  preliminary  series  indicated  the  desira- 
bility of  making  a  few  modifications  in  the  arrangement  of  the  material  which  were  embodied 
in  the  bearers  here  reported.  The  details  of  the  veneer  and  spruce  filler  types  are  shown  in 
figures  75  and  76,  respectively,  and  the  methods  used  in  thrust  loading  and  in  vertical  loading 
are  illustrated  in  figures  73  and  74,  respectively.  The  results  of  the  tests  are  shown  in  table  24: 

TABLE  24. —  Tests  on  modified  engine  bearers  (second  series}. 


Engine 
bearers 
No. 

Type. 

Weight, 
pounds. 

Moisture  con- 
tent at  test. 

Deflection 
at  maximum 
thrust  load, 
in  inches. 

Maximum 
thrust  load, 
in  pounds. 

Deforma- 
tion at 
maximum 
vertical 
load,  in 
inches. 

Maximum  ver- 
tical load,  in 
pounds. 

1 

2 
3 

1  All-  veneer  (grain  of  faces  horizontal)  .  .  . 
_•  T'  I  — 

f         6.88 
7.27 
I        7.30 

13.0 
13.4 
13.2 

2.81 
1.75 
2.25 

1,430 
1,360 
1,580 

0.63 
.61 
.49 

11,  560 
12,260 
11,800 

Average                      

7.  15 

13.2 

2.27 

1,457 

.58 

11,  873 

4 
5 

>  All-  veneer  (grain  of  faces  vertical)  .... 

f        7.54 
I         7.  11 

12.8 
11.8 

2.12 

2.42 

1,940 
1.850 

.40 

.56 

11,  360 
11,  540 

6 

I        7.40 

13.6 

1.65 

2.000 

.45 

10,  500 

Average  

7.35 

12.7 

2.06 

1,930 

.47 

11,133 

7 
8 
9 

[Plywood  with  spruce   filler   (grain  of 
faces  horizontal). 

f        7.10 
7.14 
I        7.26 

12.3 
•   12.5 
12.2 

2.04 

1.82 
1.84 

1,850 
1,720 
1,690 

.38 
.48 
.50 

12,500 
17,000 
16,000 

Average                    

7.17 

12.3 

1.90 

1,753 

.45 

15,  167 

10 
11 

12 

[Plvwood  with   spruce   filler  (grain  of 
faces  vertical). 

^x 

f        7.12 
7.20 
I        7.02 

11.6 
11.7 
12.1 

1.88 
1.62 
2.45 

1,960 
1,910 
2,150 

.60 
.49 
.67 

16,500 
16,500 
15,  430 

/ 

Averaee 

7.  11 

11.8 

1.98 

2,007 

.59 

16,  143 

' 

31 

>*-~-  f-— 

/ 

ifqe  9< 

'.» 


bflB    fli- 


vi.Kf]%  wolf  of  i  no<f»  '>!)«rii  nosd  ovarf  ftte^)  ' 
orfT     .sl/rjiqa  ttei  bn«  trfgh  m  I>»qqmw 
baidBoh  ud  ji«n  gasftftite  buioierml  form  noi^ 

• 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


Fig.  73.— Strength  tests  of  engine  bearers :  Method  of  testing  for  thrust  loading. 


138 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Fig.  74.— Strength  tests  of  engine  bearers:  Method  of  testing  for  vertical  loading. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


139 


.  y,£.J,    Graft?  of  face. 
r7or/z.onfa/ 


Wos.  4-,  3~;  6;  Gra/rr   of  face 
p//es    [/es-f/co./ 


Fig.  75. — Engine  bearers,  all-veneer  type. 
f/V/er 


l/err/ca/ 


Fig.  76.— Engine  bearers,  spruce-filler  type. 


;-   —Spruce 
Fitter 


£  l/eneer 
Each  P/y 

//7C/" 


It  is  to  be  noted  that  the  spruce-filler  type  was  somewhat  superior  to  the  other  in  thrust 
loading  and  much  superior  in  vertical  loading ;  also  that  the  bearers  of  the  former  type  with 
the  face  grain  of  the  plywood  vertical  were  superior  to  those  with  the  face  grain  horizontal. 
It  may  be  mentioned  that  these  particular  bearers  were  designed  to  take  vertical  loading  only. 


140  AIRCRAFT  DESIGN  DATA.  Note  12. 

TESTS   ON    BAKELIZED   CANVAS  (MICARTA). 

In  connection  with  tests  on  substitutes  for  wood  in  aircraft  construction  several  series  of 
tests  were  made  upon  micarta  members.  Reference  has  already  been  made  to  tests  upon 
spruce  struts  covered  with  micarta.  Tests  were  also  made  on  hollow  longerons  of  micarta  and 
on  a  few  samples  of  micarta  wing  spars. 

Several  tubes  of  micarta  were  submitted  for  test  as  substitutes  for  wood  in  longerons. 
These  tubes  were  hollow,  36  inches  long,  1  inch  square  outside,  and  made  up  of  6  plies  of  canvas  , 
the  walls  being  about  one-eighth  inch  thick.  For  comparison  a  number  of  spruce  sticks  1  inch 
square  and  36  inches  long  were  cut  from  a  plank  selected  at  random  and  comparative  tests 
made  upon  the  tubes  and  sticks.  The  tests  show  the  following  properties,  compared  to  spruce 
(moisture  about  10  per  cent): 

1  .  Modulus  of  elasticity  about  two-thirds  that  of  spruce. 

2.  Fiber  stress  at  elastic  limit  in  bending  about  four-fifths  that  of  spruce. 

3.  Tensile  strength  half  that  of  spruce. 

4.  Specific  gravity  three  times  that  of  spruce. 

5.  Compression  parallel  to  the  grain,  elastic  limit,  one-half  that  of  spruce. 

6.  Compression  parallel  to  the  grain,  maximum  load,  three  times  that  of  spruce. 
Impact  tests  upon  several  longerons  and  impact  tests  made  upon  several  samples  of  I-beam 

and  hollow  section  wing  spars-  of  micarta  indicate  that  this  material  is  superior  to  average 
spruce  (about  10  per  cent  moisture)  in  the  following  properties:  (a)  Fiber  stress  at  the  elastic 
limit  in  impact;  (6)  elastic  resilience  in  impact  (much  superior). 

The  cause  of  the  much  greater  elastic  resilience  lies  not  only  in  the  higher  fiber  stress  at 
the  elastic  limit  but  also  in  the  lower  modulus  of  elasticity  in  impact. 

In  general,  micarta  can  not  be  considered  as  a  substitute  for  spruce  in  aircraft  construction. 

TREATMENTS  FOR  PREVENTING  CHANGES  IN  MOISTURE. 

Several  long  series  of  tests  have  been  conducted  in  the  hope  of  finding  some  means  of 
preventing  changes  of  moisture  in  finished  parts  with  changing  weather  conditions. 

The  first  series  had  to  do  principally  with  varnishes  of  the  so-called  waterproof  type. 
Yellow  birch  blocks  were  given  a  coat  of  silex  filler  and  then  three  coats  of  the  varnish  under 
test.  In  some  cases  the  varnish  was  applied  with  a  brush  and  in  others  the  blocks  were  dipped  . 
Some  of  the  specimens  were  dried  in  the  air  between  coats  and  others  were  baked.  After  the 
final  coat  had  set  the  blocks  were  hung  in  a  humidity  chamber  in  which  the  relative  humidity 
was  95  per  cent  and  the  temperature  between  75  and  80  degrees  F.  The  blocks  were  weighed 
at  intervals  to  determine  the  absorption  of  moisture.  It  was  found  that  the  absorption  varied 
widely  among  the  different  varnishes  and  that  baking  improved  some  varnishes  while  increasing 
the  rate  of  moisture  transmission  through  others.  The  absorption  in  17  days  varied  from  a 
minimum  of  4.36  grams  per  square  foot  of  surface  to  a  maximum  of  26.8  grams  per  square  foot 
of  surface.  The  specimen  showing  the  least  absorption  happened  to  be  one  which  had  been 
dipped  and  air  dried,  while  the  one  showing  the  greatest  absorption  happened  to  be  brushed 
and  baked.  The  tests  showed  not  only  the  great  variability  in  moisture  resistance  among 
good  varnishes  but  showed  also  that  the  moisture  resistance  was  in  all  cases  increased  by  increas- 
ing the  number  of  coats  of  varnish  applied.  Table  25  shows  the  absorption  of  water  by  speci- 
mens given  various  miscellaneous  treatments.  The  absorptions  at  17  days  are  comparable  with 
the  figures  just  quoted.  None  of  the  treatments  furnished  the  desired  water  resistance. 


p 
.v;Lno  ^nibjsol  Uoitaav  odn)  <>J  Iwrrgie'*!)  -WN  - 


Note  12. 


AIRCEAFT  DESIGN  DATA. 


141 


TABLE  25.— Humidity  tests  of  miscellaneous  treatments. 

Wood,  yellow  birch:  Average  thickness,  0.6  inch;  average  width,  4  inches;  average  length,  8  inches;  average  surface 
area,  0.54  square  foot;  average  weight,  0.49  pound  air  dry;  average  volume,  0.011  cubic  foot. 


Treatment. 

Number 
of  speci- 
mens 
aver- 
aged. 

Average    absorption    in    grams    per 
square  foot  of  surface  in— 

3  days. 

10  days. 

17  days. 

1.  Muslin  glued  with  Le  Page's  Cold  Glue,  4  coats  of  airplane  dope,  and  2 
coats  of  airplane  gray  enamel  

2 
2 

2 

2 
2 
5 
2 
2 

2 
2 
1 

2 
2 

2 
1 
10 

1.85 
1.68 

1.80 
1.43 
1.55 
2.60 
1.97 
2.22 

4.20 
3.56 

4.86 

6.04 
6.55 

14.0 
21.6 
20.5 

4.36 
4.49 

4.79 
5.36 
5.11 
6.21 
6.75 
5.79 

10.31 
11.90 
13.40 

16.72 
23.9 

36.7 

7.39 

7.98 

8.23 
8.30 
8.35 
10.  23 
10.36 
9.93 

15.44 
17.88 
20.7 
25.4 

2.  Three  brush  coats  of  orange  shellac  

3.  Paste  filler  (silex),  1  coat  airplane  gray  undercoat,  3  coats  airplane  gray 
enamel  (Adams  &  Elting  Co  )  

4.  Paste  filler  (silex),  2  brush  coats  orange  shellac,  2  brush  coats  Lowe  Bros. 
Finishing  Varnish  V  801  

5.  Paste  filler  (silex),  2  coats  of  Hampden's  W.  P.  Varnish  No.  1  and  1  coat 
of  Lowe  Bros.  Marine  Spar  

6.  Paste  filler,  2  coats  of  white  lead,  linseed  oil,  and  lampblack,  1  coat  of 
rubbing  varnish  ,  4  coats  of  spar  varnish  

7.  Two  brush  coats  of  orange  shellac  and  2  brush  coats  of  Lowe  Bros.  Finish- 
ing Varnish  V  801  

8.  Two  brush  coats  of  orange  shellac  and  3  brush  coats  of  Lowe  Bros.  Finish- 
ing Varnish  V  801  

9.  Wood  dyed  alternating  the  two  following  solutions:  No.  1  —  100  gr.  aniline 
hydrochloride,  40  gr.  ammonium  chloride,  650  gr.  water.  No.  2  —  100 
gr.  copper  sulphate,  50  gr.  potassium  chlorate,  615  gr.  water;  washed 
with  soap  and  water  and  thoroughly  rubbed  with  vaseline;  3  coats  of 
Lowe  Bros.  Marine  Spar  Varnish  were  added  

10.  Four  brush  coats  of  Toch  Bros.  1017  Marine  Varnish  thinned  with  turpen- 
tine   

11.  One-half  hour  vacuum  and  1  hour  atmospheric  pressure  (Special  Varnish, 
Adams  &  Elting  Co  )  

12.  One-half  hour  vacuum  and  1  hour  atmospheric  pressure  (Toch  Bros.  No. 
1017  M.  S.  Preservative  

13    Hot  and  cold  treatment  with  paraffin  dissolved  in  gasoline 

14.  Five  applications  of  hot  boiled  linseed  oil  and  2  coats  of  prepared  wax, 
each  coat  applied  at  intervals  of  not  less  than  4  hours  and  each  thor- 
oughly rubbed  

47.6 

15    Same  as  9  except  that  no  varnish  was  applied 

16    Plain  yellow  birch  panels  no  treatment     .       ... 

42.5 

51.9 

Some  conclusions  not  already  mentioned  follow: 

(1)  A  more  effective  coating  may  be  secured  by  dipping  than  by  hand  brushing. 

(2)  Cellulose  varnishes  are  not  as  durable  as  oil  varnishes. 

(3)  Linseed  oil  and  wax  treatments  are  not  effective  in  keeping  out  moisture. 

(4)  All  the  varnishes  tested  were  somewhat  affected  by  water,  ^including  those  that  do 
not  turn  white  as  well  as  those  which  do. 

(5)  Very  resistant  coatings  may  be  secured  by  using  certain  rubbing  varnishes  followed 
by  top  coats  of  spar  vainish  as  a  protection,  also  by  using  certain  linseed  oil  varnishes,  covered 
by  a  more  durable  China  wood  oil  varnish. 

Tests  were  also  conducted  on  electroplated  metal  coatings  and  on  vulcanized  rubber  coat- 
ings. Both  of  these  types  of  coating  are  extremely  resistant  to  the  penetration  of  moisture, 
so  long  as  they  remain  intact.  The  metal  coating  in  particular,  however,  is  rather  delicate 
and  does  not  adhere  to  the  wood.  The  vulcanized  rubber  coatings  were  about  an  eighth  of  an 
inch  thick  and  would  probably  be  quite  satisfactory  from  the  standpoint  of  durability. 

Of  all  the  coatings  upon  which  experiments  were  made,  an  aluminum  leaf  coating  appears 
to  be  the  most  satisfactory  from  the  standpoint  of  resistance  to  moisture  penetration  com- 
bined with  general  feasibility.  This  coating  consists,  in  effect,  of  aluminum  leaf  laid  over  the 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


surface  between  layers  of  varnish,  just  as  sign  painters  lay  on  leaf  over  size.  The  leaf  itself 
has  no  wearing  strength  and  the  coating  has  just  the  durability  and  wear  resistance  of  the 
coats  of  varnish  and  enamel  placed  over  the  leaf.  The  resistance  of  the  leaf  coating  to  the 
passage  of  moisture  is  very  remarkable  indeed,  as  will  be  seen  from  a  study  of  table  26  and 
figure  81,  which  present  comparable  data  on  several  kinds  of  aluminum  leaf  coatings  and  several 
common  kinds  of  finish. 

TABLE  26. — Humidity  tests  of  metal  leaf  coatings. 


Treatment. 

Number 
of  speci- 
mens av- 
eraged.1 

Average  absorption  in  grams  per  square  foot  of  surface  for  — 

3  days.           10  days. 

17  days. 

24  days. 

31  days. 

Silex  filler,  gold  size,  aluminum  leaf,  and  3  coats  of 
E.  P.  black  lacquer  

2 
3 
1 
10 

2 
10 

-0.  210 
0.218 
0 
1.28 

14.0 
20.5 

0.084 
0.445 
0.252 
4.56 

36.7 
42.5 

0.168 
0.805 
0.420 
7.29 

47.6 
51.9 

-0.042 

0.  461 

Silex  filler,  1  coat  of  rubbing  varnish,  gold  size,  imita- 
tation  gold  leaf  1  coat  Valspar 

Silex  filler,  1  coat  of  rubbing  varnish,  gold  size,  alu- 
minum leaf  1  coat  Valspar         .                

Silex  filler,  3  brush  coats  Hampden's  waterproof  var- 
nish No.  2  

5  applications  of  linseed  oil  applied  hot  and  2  coats  of 
wax 

No  treatment  

1  Average  data  on  yellow  birch  panels. 

Thickness inch . .  0. 60 

Width inches. .  3. 960 

Length do. .  -  8. 000 

Surface square  feet . .    .  540 

Weight  (air-dry) pound . .    .490 

Volume cubic  feet . .    .011 

It  has  been  found,  in  actual  practice,  that  the  process  is  entirely  workable,  and  very  good 
results  have  already  been  secured  from  its  use. 

The  following  instructions  explain  in  detail  the  method  of  applying  aluminum  leaf  to 
propellers.  The  same  method  could  be  used  in  coating  other  aircraft  parts  if  it  were  found 
desirable  to  do  so. 

INSTRUCTIONS  FOR  APPLYING  ALUMINUM  LEAF  TO  AIRCRAFT  PROPELLERS. 

The  leaf  used  in  this  process  is  exceedingly  thin  and  light,  there  being  probably  12,000 
to  15,000  leaves  per  inch  which  makes  it  appear  difficult  to  handle.  If  the  instructions  are 
carefully  followed,  however,  the  leaf  may  be  easily  and  thoroughly  applied. 

Preparation. — It  is  important  to  provide  a  perfectly  smooth  surface  over  which  to  apply 
the  coating.  The  surface  should  be  sanded  perfectly  smooth  and  be  free  from  all  tool  marks 
or  other  imperfections.  The  bolt  holes  at  the  hub  should  be  plugged  with  corks  which  should 
be  cut  off  flush  and  finished  in  the  same  manner  as  the  rest  of  the  surface. 

Filling. — For  open-grained  woods  a  coat  of  filler  consisting  of  83  per  cent  liquid  and  17 
per  cent  silex  should  be  used.  The  liquid  should  consist  of  77  per  cent  airplane  spar  varnish 
and  23  per  cent  turpentine.  The  silex  should  pass  a  200-mesh  sieve. 

The  filler  should  be  applied  to  the  wood  and  allowed  to  flatten,  after  which  it  should  be 
rubbed  off  across  the  grain  so  as  to  thoroughly  fill  the  pores.  The  filler  should  dry  at  least 
24  hours,  after  which  it  should  be  sanded  lightly. 

TJIJ       .  <-"  V 

aril  -mo  bial  la^f  mifaiaufU  to  .*•>*&»  ni  ,etet 


Note  12.  AIRCRAFT  DESIGN  DATA.  143 


Shellac  varnish  undercoating. — The  shellac  varnish  should  consist  of  four  and  one-half 
pounds  of  orange  shellac  gum  in  one  gallon  of  clean,  neutral,  denatured  alcohol. 

This  varnish  should  be  applied  evenly  over  the  surface  of  the  propeller  and  allowed  to  dry 
three  or  four  hours,  after  which  it  should  be  sanded  lightly. 

Size. — The  size  should  consist  of  75  per  cent  airplane  spar  varnish  and  25  per  cent  turpen- 
tine. It  is  suggested  that  a  small  amount  of  Prussian  blue  in  Japan  be  added  to  the  varnish 
to  give  it  a  color,  so  that  spots  subsequently  left  uncovered  by  the  leaf  will  be  readily  visible. 

This  size  should  be  brushed  evenly  over  the  surface  as  sparingly  as  possible  and  allowed 
to  dry  until  a  tack  is  reached,  which  will  permit  the  handling  of  the  propeller  immediately 
after  the  application  of  the  leaf.  The  time  will  vary  with  the  varnish  arid  the  kind  of  a  day. 
The  varnish  should  probably  dry  an  hour  and  a  half  on  a  light  dry  day  or  in  a  heated  building 
in  the  winter  time,  but  a  longer  time  may  be  required  on  cloudy  or  damp  days.  This  is  a  very 
important  point  and  should  be  carefully  considered  as  the  coating  hardens  very  slowly  after 
the  leaf  is  applied. 

Care  should  be  exercised  so  as  not  to  produce  fatty  edges  or  runs  in  applying  the  size. 
If  they  occur,  the  leaf  will  be  easily  rubbed  from  the  surf  ace  in  handling  the  blade. 

It  has  been  found  convenient  to  size  one  side  of  the  blade  at  a  time;  that  is,  the  front  or 
back  of  the  blade.  This  is  a  convenience  in  applying  the  leaf  later. 

Aluminum  leaf. — After  the  size  has  reached  the  right  tack  the  leaf  should  be  applied  very 
rapidly  over  the  surface,  and  after  the  sized  surface  has  been  entirely  covered  the  leaf  should 
be  patted  down  with  the  palm  of  the  hand  or  with  a  pad  of  cotton,  after  which  the  rough  edges 
should  be  rubbed  away  (see  fig.  79b).  Any  points  not  covered  with  leaf  should  be  coated  by 
applying  a  small  piece  of  leaf  to  the  spot  with  the  fingers.  The  coating  should  be  rubbed  well 
with  a  piece  of  cotton  which  has  been  dipped  in  aluminum  powder.  This  will  insure  the  leaf 
sticking  securely  over  the  entire  surface  and  will  fill  any  small  holes  not  already  filled. 

Aluminum  leaf  comes  in  packs  containing  500  leaves.  The  pack  is  divided  up  into  10  or 
20  books  containing  50  or  25  leaves,  respectively.  The  metal  leaf  is  placed  between  the  pages 
of  these  books  and  comes  in  4-inch,  4^-inch,  5-inch,  or  5^-inch  squares. 

It  has  been  found  best  to  apply  the  leaf  directly  from  the  book  by  turning  back  the  first 
page  of  the  book  halfway,  holding  the  same  between  the  first  and  second  fingers  of  the  right 
hand  (see  fig.  78a) .  The  book  itself  should  be  held  between  the  thumb  and  fingers  and  in 
such  a  way  that  the  back  of  the  hand  will  be  toward  the  work  when  the  leaf  is  applied,  the 
book  being  given  a  slight  bend  to  prevent  the  corners  of  the  leaf  from  drooping.  The  end  of 
the  leaf  exposed  by  turning  back  the  first  page  of  the  book  should  be  placed  against  the  surface 
to  be  coated  and  held  securely  in  place  by  the  left  hand  (see  fig.  78b).  The  sheet  held  between 
the  first  and  second  fingers  should  be  drawn  back  so  as  to  allow  the  whole  leaf  to  come  in  contact 
with  the  surface  (see  fig.  79a).  The  next  sheet  should  be  applied  in  a  like  manner,  lapping 
edges  with  the  first,  and  so  on.  The  best  results  will  be  obtained  if  the  gilder  works  in  one 
direction  with. each  row  of  leaf;  that  is,  from  left  to  right.  If  this  be  done,  it  will  aid  considerably 
in  completing  and  smoothing  off  the  surface. 

It  is  suggested  that  in  turning  the  pages  of  the  books  the  back  of  the  book  be  held  between 
the  first  two  fingers  of  the  left  hand  (see  fig.  77a).  The  leaves  from  which  the  leaf  has  been 
removed  should  be  turned  back  and  held  between  the  thumb  and  first  finger  of  the  left  hand. 
The  next  sheet  of  paper  may  then  be  turned  back  exposing  one-half  of  the  next  leaf.  The 
operation  of  changing  the  book  from  left  to  right  hand  is  shown  in  figure  77b. 

Large  hub  hole. — The  large  hub  hole  should  receive  the  same  treatment  as  the  rest  of  the 
propeller.  In  applying  the  leaf  to  the  hub  hole  it  has  been  found  convenient  to  cut  the  books 


144 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


Fig.  77. — Aluminum  leaf  coating,     (a)  Method  used  in  turningjpage  of  book,     (b)  Transferring  book  from  left  to 

right  hand. 


Fig.  78. — Aluminum  leaf  coating,     (a)  Method  of  holding  book  when  applying  leaf,     (b)  First  operation  in  laying  leaf. 


Fig.  79. — Aluminum  leaf  coating,     (a)  Second  operation  in  laying  leaf,     (b)  Smoothing  off  surface  after  application 

of  leaf. 


Fig.  80. — Aluminum  leaf  coating,    (a)  Applying  leaf  to  large  hub  hole,    (b)  Smoothing  off  leaf  in  large  hub  hole. 


Note  12. 


AIRCRAFT  DESIGN  DATA. 


145 


of  leaf  up  into  about  1-inch  strips  of  leaf  and  paper  and  drop  them  vertically  into  the  opening 
and  bring  into  contact  with  the  size  (see  fig.  80a).  After  the  entire  surface  of  the  hole  has 
been  covered  the  leaf  should  be  patted  into  place  with  a  wad  of  cotton  attached  to  the  end  of  a 
stick  (see  fig.  80b). 

Small  hub  holes.—  These  holes  should  be  simply  corked  up  with  ordinary  corks,  the  tops  of 

which  should  be  cut  off  flush  with  the  surface  of  the  propeller  and  covered  with  the  regular  finish. 

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Fig.  81. — The  comparative  effectiveness  of  various  coatings  in  moisture-proofing  wood. 

95  to  100  per  cent. 

Shellac  color  varnish. — After  the  application  of  the  leaf  two  coats  of  shellac  color  varnish 
should  be  applied.  This  varnish  should  be  made  as  described  under  the  heading  of  "Shellac 
varnish  undercoat,"  except  that  enough  color  should  be  added  to  produce  a  mahogany  color. 
Four  or  5  per  cent  of  Bismark  brown  in  the  shellac  varnish  gives  about  the  right  color.  The 
amount  of  this  material  to  get  the  best  results  should  be  determined  by  trial.  The  varnish 
should  dry  three  or  four  hours  before  rubbing  or  recoating. 
98257— 19— No.  12 10 


146  AIRCRAFT  DESIGN  DATA.  Note  12. 

Each  coat  of  shellac  should  be  rubbed  down  lightly  between  coats  without  the  use  of  oil. 

Finishing  varnish. — A  final  flowing  coat  of  airplane  spar  varnish  should  be  applied  and 
allowed  to  dry  about  48  hours.  This  coating  should  not  be  rubbed  or  sanded. 

Estimated  time  required  to  coat  a  propeller. — The  time  required  to  apply  the  leaf  to  a  propeller 
should  not  be  more  than  40  or  50  minutes.  This  time  could  be  reduced  after  the  finisher  becomes 
more  experienced.  The  estimated  time  required  for  applying  the  complete  finish  described 
in  the  foregoing  paragraphs  would  be  in  the  neighborhood  of  8  or  10  hours,  and  the  total 
time  required  for  drying  the  various  coats  about  90  hours.  The  total  time  required  for  the  total 
operation  would  probably  be  in  the  neighborhood  of  100  hours. 

Modification  of  aluminum  leaf  spirit  varnish  process. — It  might  be  desirable  in  some  cases 
to  use  oil  varnishes  or  enamels  in  lieu  of  the  shellac  described  above.  This  may  be  done  and 
satisfactory  results  obtained.  In  case  oil  varnishes  are  substituted,  it  is  possible  that  a  more 
durable  coating  will  be  obtained.  It  requires  a  much  longer  time  to  apply  the  finish  because 
of  the  greater  time  required  for  the  oil  varnishes  to  dry.  Each  coat  of  varnish  should  dry  at 
least  72  hours  before  recoating. 


,ATA<I  HOiasa-TiAflOXIA 


APPENDIX. 

For  convenient  reference,  specifications  for  the  determination  of  the  moisture  content  in 
wood  and  for  the  determination  of  the  specific  gravity  of  wood  are  embodied  in  the  appendix. 

THE  DETERMINATION  OF  MOISTURE  CONTENT  IN  WOOD. 

SELECTION    OF    TEST    SPECIMENS. 

1.  Short  pieces  of  wood  dry  out  much  more  rapidly  than  longer  ones.     In  order  to  reduce 
the  time  required  for  drying,  the  length  of  the  test  specimen  in  the  direction  of  the  grain  should 
usually  be  about  1  inch  or  not  more  than  enough  to  give  a  volume  of  from  5  to  25  cubic  inches. 

TESTS. 

2.  Having  selected  a  representative  piece  of  material  for  a  test  specimen,  the  procedure 
for  determining  the  moisture  content  is  as  follows: 

3.  Immediately  after  sawing  remove  all  loose  splinters  and  weigh  the  test  specimen.     It 
is  important  that  the  weight  be  taken  immediately  after  sawing,  since  the  wood  is  subject  to 
moisture  changes  on  exposure  to  the  air.     The  degree  and  rapidity  of  change  are  dependent 
on  the  moisture  content  of  the  piece  and  the  conditions  of  the  air  to  which  it  is  exposed. 

4.  Put  the  test  specimen  into  a  drying  oven  and  dry  at  approximately  212°  F.  (100°  C.) 
to  constant  weight.     This  usually  requires  three  to  five  days.     Specimens  placed  in  the  oven  for 
drying  must  be  open  piled  to  allow  free  access  of  air  to  all  parts  of  each  piece. 

5.  Weigh  the  test  specimens  immediately  after  removing  from  the  oven. 

6.  The  loss  in  weight  expressed  in  per  cent  of  the  dry  weight  is  the  percentage  moisture 
content  of  the  wood  from  which  the  test  specimen  was  cut. 

CW"  —  D) 
Percentage  moisture  =  — r\ X 100 

\V  =  original  weight  as  found  under  paragraph  3. 

D=i  oven-dry  weight  as  found  under  paragraph  5. 

I      -^  11 

ACCURACY. 


7.  In  order  to  insure  good  results,  the  weight  should  be  correct  to  within  at  least  one-half 

of  1  per  cent. 

THE  DETERMINATION  OF  SPECIFIC  GRAVITY  OF  WOOD. 

GENERAL. 
.«H< 

1 .  The  specific  gravity  (or  density)  of  all  woods  used  in  aircraft  construction  shall  be  deter- 
mined, when  required,  in  accordance  with  this  specification.     Method  A  shall  be  used  whenever 
possible. 

SELECTION   OF   TEST    SPECIMENS. 

2.  Short  pieces  of  wood  dry  out  much  more  rapidly  than  longer  ones.     In  order  to  reduce 
the  time  required  for  drying,  the  length  of  the  test  specimen  in  the  direction  of  the  grain  should 
usually  be  about  3  centimeters. 

147 

.Klft! 


148 


AIRCRAFT  DESIGN  DATA. 


Note  12. 


METHOD  A. 

3.  Having  selected  a  representative  piece  of  material  for  a  test  specimen,  the  procedure  is 
as  follows: 

4.  Immediately  after  sawing  remove  all  loose  splinters  and  put  the  test  specimen  into  a 
drying  oven  and  dry  at  about  212°  F.  (100°  C.)  to  constant  weight.     This  usually  requires 
three  to  five  days.     Specimens  placed  in  the  oven  for  drying  must  be  open  piled  to  allow  free 
access  of  air  to  all  parts  of  each  piece. 

5.  Weigh  the  test  specimen. 

6.  Determine  the  volume  of  the  oven-dry  specimen  preferably  by  the  method  described  in 
paragraphs  9  to  12. 

7.  Specific  gravity  =  y 

M  =  oven-dry  weight  in  grams  as  determined  under  paragraphs  4  and  5. 

V  =  oven-dry  volume  in  cubic  centimeters  as  determined  under  paragraph  6. 


S(l*  A* 
o^  tr>9i<fw8  e      II 

Jnabneqeb  n     II 
;x 


.0  °OOI)  .1  c«  vbi 


Fig.  82. — Determination  of  specific  gravity  of  wood. 


REDUCTION   FACTORS. 

•!*>)':>  L    ~       .  .  .    .  . 

8.  Onemch  =  2.54  centimeters;  1  ounce  =  28.4  grams;  1  cubic  mch  =  16.4  cubic  centimeters; 

1  pound  =  454  grams. 


DETERMINATION    OF    VOLUME. 


9.  After  the  oven-dry  weight  has  been  obtained  dip  the  test  specimen  in  hot  paraffin  and 
allow  it  to  cool.     Scrape  off  any  surplus  paraffin  which  adheres  to  the  specimen. 

10.  The  volume  of  the  test  specimen  is  found  by  determining  the  weight  of  water  it  dis- 
places when  immersed,  as  shown  in  figure  82.     This  weight  in  grams  is  numerically  equal  to 
the  volume  of  the  specimen  in  cubic  centimeters. 


Note  12.  AIRCRAFT  DESIGN  DATA.  149 

11.  It  is  important  that  the  determination  of  the  volume  by  weighing  be  made  as  quickly 
as  possible  after  the  immersion  of  the  specimen,  since  any  absorption  of  water  by  the  specimen 
directly  influences  the  accuracy  of  the  result.     By  estimating  the  volume  of  the  specimen  and 
placing  approximately  the  required  weights  on  the  plan  before  the  specimen  is  immersed  the 
time  necessary  for  balancing  may  be  reduced  to  a  minimum. 

12.  To  determine  the  volume,  a  container  holding  sufficient  water  for  the  complete  sub- 
mergence of  the  specimen  is  placed  on  one  pan  of  a  balance  scale.     The  container  and  water  are 
then  balanced  with  weights  added  to  the  other  scale  pan.     By  means  of  a  sharp-pointed  rod, 
shown  in  figure  82,  the  specimen  is  held  completely  submerged  and  not  touching  the  container 
while  the  scales  are  again  balanced.     The  weight  required  to  balance  is  the  weight  of  water 
displaced  by  the  specimen,  and,  if  in  grams,  is  numerically  equal  to  the  volume  of  the  specimen 
in  cubic  centimeters. 

13.  The  sharp-pointed  rod,  by  means  of  which  the  specimen  is  held  in  position,  should  be 
of  as  small  diameter  as  possible.     Care  should  be  taken  not  to  lower  the  specimen  into  the 
water  to  a  much  greater  depth  than  required  to  completely  submerge  it;  otherwise  the  weight 
of  water  displaced  by  the  rod  will  affect  the  accuracy  of  the  result. 

ACCURACY. 

14.  In  order  to  insure  good  results,  the  weights  and  volumes  should  be  correct  to  within 

at  least  one-half  of  1  per  cent. 

METHOD  B. 

15.  The  following  method  of  determining  the  specific  gravity  may  be  used  when  the  appa- 
ratus required  by  test  A  is  not  available. 

16.  Select  the  test  specimen  as  in  paragraph  2. 

17.  Dry  the  specimen  as  in  paragraph  4. 

18.  Cut  the  oven-dried  specimen  while  hot  to  a  standard  volume  of  not  less  than  80  cubic 
centimeters  so  that  its  volume  may  be  accurately  determined  by  measurement. 

19.  Weigh  the  oven-dried  specimen  while  hot  and  record  its  weight  in  grams.     This  weight 
must  be  accurate  to  within  one-half  of  1  per  cent. 

20.  Determine  the  volume  in  cubic  centimeters  of  the  oven-dried  specimen  while  hot  by 
measuring  each  edge  in  centimeters  and  taking  measurements  to  the  nearest  one-half  millimeter. 

0       .  .         M    weight  in  grams 

20.  Specific  gravity--— • 


WASHINGTON  :  GOVERNMENT  PRINTING  OFFICE  :  1919 


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